How I Improved My Shell Load Time With a Lazy Load

If you’ve used multiple tools, languages, build environments, etc.. as a CLI (command line utility), then you’ve probably experienced slow load times for your shell environment at some point. A simple way to determine which CLI is the slowest is to measure startup time. After identifying the slowest CLI tool with a profile, lazy loading that tool to the environment is the simplest approach to try when attempting to improve load times for a shell environment. I will provide an example specific to lazy loading zsh, but the general idea can be applied to other environments.

0. Diagnosing the problem

time zsh -i -c exit

• When my shell starts feeling a little sluggish, I use time to get an accurate measure of what I’m feeling.
• On unix systems with a shell, you’ll probably have some version of the time utility. To get the specific information for your specific shell, I recommend reading man time. The utility is useful for measuring CLI program run times.
• In our example, we are simply passing zsh to time to see how long it takes for it to start and stop.
• The -i flag forces zsh to be in interactive mode.
• The -c flag passes any commands to zsh in interactive mode. Remember to use native tools or tools in your PATH.
• Output:

Saving session...completed.
zsh -i -c exit  0.66s user 0.72s system 89% cpu 1.543 total

• At first the results don’t seem so bad, but we can get a more granular look at what is being loaded.
• In your .zshrc add the following:

# beginning of .zshrc
zmodload zsh/zprof
 
# contents of .zshrc must be in between
 
# end of .zshrc
zprof

• zmodload loads the built in zsh modules on launch if added to the .zshrc as above.
• The zsh/zprof module profiles everything that is loaded on start in zsh.
• On startup, zsh will output something similar to the output below. Once the source of the slow load time is identified, I usually comment out zmodload zsh/zprof and zprof in my .zshrc for future use.
• Output:

num  calls                time                       self            name
-----------------------------------------------------------------------------------
 1)    1         835.86   835.86   80.69%    309.23   309.23   29.85%  nvm_auto
 2)    2         480.08   240.04   46.35%    278.23   139.12   26.86%  nvm
 3)    1         170.17   170.17   16.43%    145.25   145.25   14.02%  nvm_ensure_version_installed
 4)   26         104.45     4.02   10.08%     78.22     3.01    7.55%  _omz_source
 5)    4          58.36    14.59    5.63%     58.36    14.59    5.63%  compaudit
 6)    1          31.50    31.50    3.04%     31.29    31.29    3.02%  nvm_die_on_prefix
 7)    2          87.03    43.51    8.40%     28.66    14.33    2.77%  compinit
 8)    1          46.55    46.55    4.49%     25.39    25.39    2.45%  nvm_is_valid_version
 9)    1          24.92    24.92    2.41%     24.92    24.92    2.41%  nvm_is_version_installed
10)    1          15.93    15.93    1.54%     11.14    11.14    1.08%  nvm_validate_implicit_alias
11)    1          10.13    10.13    0.98%      9.98     9.98    0.96%  _zsh_highlight_load_highlighters
12)    1           7.96     7.96    0.77%      7.96     7.96    0.77%  zrecompile
13)    1           5.81     5.81    0.56%      5.81     5.81    0.56%  test-ls-args
14)    1           5.23     5.23    0.51%      5.23     5.23    0.51%  nvm_version_greater_than_or_equal_to
15)    1           4.72     4.72    0.46%      4.72     4.72    0.46%  nvm_echo
16)    1           2.12     2.12    0.21%      2.10     2.10    0.20%  _zsh_highlight__function_callable_p
17)    1           1.67     1.67    0.16%      1.67     1.67    0.16%  regexp-replace
18)    3           1.48     0.49    0.14%      1.40     0.47    0.14%  add-zle-hook-widget
19)    9           1.37     0.15    0.13%      1.37     0.15    0.13%  is-at-least
20)   13           1.35     0.10    0.13%      1.35     0.10    0.13%  compdef
21)    1           1.16     1.16    0.11%      1.16     1.16    0.11%  colors
22)    7           0.92     0.13    0.09%      0.92     0.13    0.09%  add-zsh-hook
23)    1           0.27     0.27    0.03%      0.27     0.27    0.03%  (anon) [/Users/ankit/.oh-my-zsh/custom/plugins/zsh-autosuggestions/zsh-autosuggestions.zsh:458]
24)    4           0.21     0.05    0.02%      0.21     0.05    0.02%  nvm_npmrc_bad_news_bears
25)    6           0.18     0.03    0.02%      0.18     0.03    0.02%  is_plugin
26)    1           0.17     0.17    0.02%      0.17     0.17    0.02%  nvm_has
27)    1           0.13     0.13    0.01%      0.13     0.13    0.01%  zvm_exist_command
28)    1           0.22     0.22    0.02%      0.10     0.10    0.01%  complete
29)    3           0.09     0.03    0.01%      0.09     0.03    0.01%  is_theme
30)    1           0.08     0.08    0.01%      0.08     0.08    0.01%  (anon) [/usr/local/Cellar/zsh/5.9/share/zsh/functions/add-zle-hook-widget:28]
31)    2           0.07     0.04    0.01%      0.07     0.04    0.01%  env_default
32)    1           4.79     4.79    0.46%      0.07     0.07    0.01%  nvm_err
33)    2           0.07     0.03    0.01%      0.07     0.03    0.01%  bashcompinit
34)    1         835.90   835.90   80.69%      0.04     0.04    0.00%  nvm_process_parameters
35)    1           0.02     0.02    0.00%      0.02     0.02    0.00%  _zsh_highlight__is_function_p
36)    1           0.01     0.01    0.00%      0.01     0.01    0.00%  nvm_is_zsh
37)    1           0.00     0.00    0.00%      0.00     0.00    0.00%  _zsh_highlight_bind_widgets

• As a reminder, whenever .zshrc is changed. Saving then reloading it will ensure all changes are reflected. Reload with source .PATH/TO/.zshrc.

1. Lazy Load

• From the output of zmodload zsh/zprof, it seems nvm is the likely reason my start up time is slow. If I lazy load the right things, I could improve my zsh start up time.
• Lazy loading depends on your environment. If your local configuration is slightly different, then focusing on where your tools are loaded from is a reasonable starting point. Delaying the load of the slowest tool is the goal.
• To lazy load nvm, I added wrapper functions around the nvm, node, npm, and npx variable names to change their respective scope (a.k.a overloading variable name). zsh will see the function before something further down the PATH chain. When nvm, node, npm, or npx is explicitly called then zsh will load the resources into the current session.
• For me, adding the below functions to my .zshrc fixed my zsh load time.

# lazy lode nvm instead of through oh-my-zsh to reduce load by 50%
lazy-nvm()
{
  unset -f nvm node npm npx
  export NVM_DIR="$HOME/.nvm"
  [ -s "$NVM_DIR/nvm.sh" ] && \. "$NVM_DIR/nvm.sh"  # This loads nvm
  [ -s "$NVM_DIR/bash_completion" ] && \. "$NVM_DIR/bash_completion"  # This loads nvm bash_completion
}
 
nvm()
{
  lazy-nvm
  nvm $@
}
 
node()
{
  lazy-nvm
  node $@
}
 
npm()
{
  lazy-nvm
  npm $@
}
 
npx()
{
  lazy-nvm
  npx $@
}

2. Verification

time zsh -i -c exit

• After lazy loading nvm, my zsh startup time was cut in half.

Saving session...completed.
zsh -i -c exit  0.32s user 0.31s system 92% cpu 0.673 total

• Adding the below lines to the .zshrc again will confirm nvm isn’t loaded on start:

# beginning of .zshrc
zmodload zsh/zprof
 
# contents of .zshrc must be in between
 
# end of .zshrc
zprof

• Output:

num  calls                time                       self            name
-----------------------------------------------------------------------------------
 1)   26         109.34     4.21   71.45%     82.96     3.19   54.22%  _omz_source
 2)    2          29.39    14.70   19.21%     29.39    14.70   19.21%  compaudit
 3)    1          10.67    10.67    6.97%     10.51    10.51    6.87%  _zsh_highlight_load_highlighters
 4)    1          36.67    36.67   23.96%      7.27     7.27    4.75%  compinit
 5)    1           6.67     6.67    4.36%      6.67     6.67    4.36%  zrecompile
 6)    1           5.15     5.15    3.36%      5.15     5.15    3.36%  test-ls-args
 7)    1           2.21     2.21    1.45%      2.19     2.19    1.43%  _zsh_highlight__function_callable_p
 8)    3           1.74     0.58    1.14%      1.63     0.54    1.07%  add-zle-hook-widget
 9)    9           1.56     0.17    1.02%      1.56     0.17    1.02%  is-at-least
10)   12           1.50     0.12    0.98%      1.50     0.12    0.98%  compdef
11)    1           1.20     1.20    0.78%      1.20     1.20    0.78%  colors
12)    7           1.07     0.15    0.70%      1.07     0.15    0.70%  add-zsh-hook
13)    1           0.88     0.88    0.57%      0.88     0.88    0.57%  regexp-replace
14)    1           0.26     0.26    0.17%      0.26     0.26    0.17%  (anon) [/Users/ankit/.oh-my-zsh/custom/plugins/zsh-autosuggestions/zsh-autosuggestions.zsh:458]
15)    6           0.26     0.04    0.17%      0.26     0.04    0.17%  is_plugin
16)    1           0.19     0.19    0.13%      0.19     0.19    0.13%  zvm_exist_command
17)    1           0.11     0.11    0.07%      0.11     0.11    0.07%  (anon) [/usr/local/Cellar/zsh/5.9/share/zsh/functions/add-zle-hook-widget:28]
18)    3           0.09     0.03    0.06%      0.09     0.03    0.06%  is_theme
19)    2           0.08     0.04    0.05%      0.08     0.04    0.05%  env_default
20)    1           0.02     0.02    0.02%      0.02     0.02    0.02%  bashcompinit
21)    1           0.02     0.02    0.01%      0.02     0.02    0.01%  _zsh_highlight__is_function_p
22)    1           0.00     0.00    0.00%      0.00     0.00    0.00%  _zsh_highlight_bind_widgets

• As a reminder, whenever .zshrc is changed. Saving then reloading it will ensure all changes are reflected. Reload with source .PATH/TO/.zshrc.

• So if we run the below command:

time zsh -i -c exit && time zsh -i -c "nvm --version" exit

• The output will show an increased load time with nvm.
• bash or zsh will read the commands left to right
• time zsh -i -c exit will complete before time zsh -i -c "nvm --version" exit.

Saving session...completed.
zsh -i -c exit  0.32s user 0.30s system 92% cpu 0.670 total
Restored session: Mon Feb 17 21:34:18 CST 2025
0.40.1
 
Saving session...completed.
zsh -i -c "nvm --version" exit  0.73s user 0.90s system 99% cpu 1.641 total

	time zsh -i -c "nvm --version" exit	time zsh -i -c exit
User	0.73s	0.32s
System	0.90s	0.30s
Total	1.641	0.670

3. References

• gnu time
• zmodload
• zsh/zprof