After update to
JetBrains ReSharper 2020.2.1 Build 202.0.20200820.125348 built on 2020-08-20
ReSharper C++ 2020.2.20200820.145616
In one of my projects, reharper makes VS slow to change files with Ctrl-Tab or Header/source switch (around 70 projects, some megabytes of sources).
It doesn't affect smaller project though. also if I disable resharper it's not slow. May be you are doing something
when editor becomes active? I don't know how debug this further. Is there a profiler or something, built into resharper?