I Have the same Problem with PHP Tools (VS 2017 / 2019)
As soon as I have a PHP Project in my Soulution, all editors get very slow.
Scrolling in all Windows (Solution Explorer and Files for Example) hangs regularly for about a second.
Typing is very laggy and therefor very uncomfortable.
Problems get Bigger when Debugger is Started.
File or Project Size seem to make no difference.
Visual Studio reports dthis issue and wants to disable the Extension.
While VS is hanging, a single core just spikes at about 70% (32 Cores at 2,8GHz)
Memory seems not to be an issue (25 used of 96)
With PHP Tools disabled, each core stays well below 10 %, scrolling and typing is instant.
https://seafile.3dmedium.de/d/b365e2439d5e4403af3e/ is enough for me to reproduce
Tested version in VS2019 is 1.40.11866