Hello Jan Kruse ,

There are a few things I would check if Studio 2021 is taking such a long time to open.

First, I would suggest making sure that everything is up-to-date  (OS/drivers etc)

https://community.sdl.com/product-groups/translationproductivity/w/customer-experience/3392/how-to-part-1-trouble-shoot-unexplained-errors-and-unwanted-behaviour-in-studio-multiterm (Parts 1 and 2)

and ensuring that both Studio and MultiTerm are up to date and are added to your AntiVirus 'exclusion list'.

Next, I would look at where you have your resources stored- are these local? or stored in the cloud?

If you have cloud-based resources, do you notice a big difference if you temporarily disconnect, then restart Studio?

Do you have a project 

If the problem persists, it's worth perhaps removing installed plugins, and reinstalling one by one to see if a particular app is causing Studio to load slowly.