As I mentioned in a comment I had the same problem. Here’s what I found;
There’s a setting telling VIM where to look for completions, called complete.
:set complete
complete=.,w,b,u,t,i
this is the default value. My problem is (was actually..) the ‘i’, which scans all included files. Here are two problems, first one, finding all those files might take quite a while, especially if you, like me, have
:set path=**
Second problem, once found, they need to be read, and if you’re using a networked file system (I’m on clearcase) both finding and reading all those files might trigger cache misses, making it painfully slow.
I’ve removed the i for now, as I have a tags-file and more often than not, I also have the relevant files in my buffers (loaded or unloaded) which will be searched as a result of ‘b’ and ‘u’.
Use
set complete-=i
to remove the i from the list, note that this is local to the buffer.