MacVim hangs for minutes with hundreds of threads

Describe the bug
I'm a daily user of MacVim and my work depends on it. MacVim occasionally hangs for minutes with hundreds of threads (500 or more) when editing some specific large file(s). I had this happen several times, but only once a day at the most. It could be a syntax highlighting problem perhaps. It's difficult to reproduce exactly.

To Reproduce
Detailed steps to reproduce the behavior:

1. Run open -a MacVim Forth500.s
2. Edit Forth500.s and perhaps some other files in other windows (but don't think that matters)
3. Change Syntax to Assembly/Turbo then navigate, search, type, etc.
4. Eventually the problem occurs, but I can't tell when and why. It may happen after a while, so it's not frequent, fortunately for me.

Expected behavior
No hot laptop!

Screenshots
N/A

Environment (please complete the following information):

• MacVim version: Custom Version 8.2.2681 (171)
• OS: MacOS 10.15.7
• Terminal: N/A

Additional context
The Forth500.s file is here: https://github.com/Robert-van-Engelen/Forth500/blob/main/Forth500.s

Start by reproducing the hang with open -a MacVim Forth500.s, then edit Forth500.s while using the Assembly/Turbo syntax and navigating, searching, or typing. Compare the behavior with the supplied Forth500.s file and the reported MacVim 8.2.2681 environment; done means identifying a reliable cause and confirming that the minutes-long hang and thread growth no longer occur.