Running slowly with multiple cpp files

I have three cpp files, all connected, along with their corresponding header files, and it is getting achingly slow, and getting slower each time I add a new file. I could handle a little bit of lag, but this is really bad. Also, whenever I try to add a comment, vim freezes after I've added the first / character. It takes between 10 to 60 seconds to start up again.

I am using vim 8.2 in the windows 10 msys2 environment.

Reproduce the slowdown with three connected .cpp files and their headers in Vim 8.2 under Windows 10 MSYS2, including the freeze after typing the first / in a comment. Trace the plugin's startup and comment-insertion behavior; done means startup and editing no longer take 10–60 seconds.

Written by the indexing model from the issue text.