How do I speed up Vim's insert mode completion by disabling include-file scanning?

Explanation

Vim's complete option controls which sources are scanned when you press <C-n> or <C-p> to complete a word. By default, the i flag instructs Vim to scan all files reachable via include statements — useful for C/C++ projects but painfully slow in large codebases where headers span thousands of files.

Removing the i flag with :set complete-=i restricts completion to the current buffer, loaded buffers, and tags — all of which are already cached in memory.

How it works

• complete is a comma-separated list of single-letter flags specifying completion sources:
  • . — current buffer
  • b — other loaded buffers
  • u — unloaded buffers
  • i — included files (the slow one)
  • t — tags (ctags)
  • w — buffers in other windows
• complete-=i removes the i flag while leaving the rest unchanged
• The result is faster, more predictable completion

Example

Before (default value):

:set complete?
complete=.,w,b,u,t,i

After:

:set complete-=i
:set complete?
complete=.,w,b,u,t

Typing <C-n> in insert mode now completes instantly from buffers and tags without walking the include chain.

Tips

• Add this to your vimrc alongside :set complete-=u to also skip unloaded buffers if you prefer even tighter control
• View current sources at any time with :set complete?
• To fully understand all flags: :help 'complete'