Same experience here with 2019. With each project, Studio 2019 becomes successively slower, to the extent that a reboot becomes necessary to get back to normal speed (for two or three projects). Whilst I only work locally and never work with packages.
Particuarly slow is switching between the concordance window, to select (parts) of text and the editor window. Each switch may take more than half a minute. In 2017 and the very first iteration of 2019, it used to be instantaneous.

Hello Martin Lee ,

As you are working with GroupShare (online) there are a number of things that could be causing or contributing to the lack of performance.

The first thing I would do is check that Studio (and MultiTerm) is added to your AntiVirus 'whitelist'.

https://community.sdl.com/product-groups/translationproductivity/f/90/t/20602

-and that everything is up to date-

https://community.sdl.com/product-groups/translationproductivity/w/customer-experience/3392.how-to-part-1-trouble-shoot-unexplained-errors-and-unwanted-behavior-in-studiomultiterm

Then if the issues persist - try disabling the Verification (one by one) to see which one is causing the issue-

Would you also be able to confirm which version of GS your client is using.