WebStorm very slow on GIT operations with NPM-packages and TypeScript; Lags; Wrong error-messages for optional type-parameters.

We wanted to switch from VSC to WebStorm and have evaluated it for the last weeks. 

WebStorm 2020.3
Build #WS-203.5981.135, built on November 27, 2020
Runtime version: 11.0.9+11-b1145.21 amd64
VM: OpenJDK 64-Bit Server VM by JetBrains s.r.o.
Windows 10 10.0
GC: ParNew, ConcurrentMarkSweep
Memory: 1961M
Cores: 8

The machine has a total memory of 16GB, of which 10 are used when 3 instances of WebStorm are running.

Slow GIT-operations

We are developing TypeScript Frontend- and Backend-Packages and use npm link extensively.   

Due to the non-support of TS declaration maps (see following link to post) we add source directories to our project. 

https://intellij-support.jetbrains.com/hc/en-us/community/posts/360004391019-Typescript-declaration-maps-declarationMap-support

This seems to cause performance problems. Sometimes the Dialog "Performing VCS Refresh" takes minutes to finish or seems to hang. I've found several entries in the forums on that topic.

Sometimes updating the "Default Changelist" takes very long, too.

Operations like merging take 2-4 minutes although they take a few seconds in the console.

Wrong TS-errors

Another  problem are wrong error messages for optional TS type-Parameters, e.g. "class A<t=any>".

Declarations like "let a: A<Person>" cause the error message that "A" is not generic. TSC itself compiles it without problems. 

Laggy IDE

Generally, WebStorm seems laggy under Windows 10. Sometimes the cursor freezes for almost a second, we see typing and intellisense delays.