Thanks for providing the video. What desktop environment are you using in this case?
Do you see the same issue if you launch RStudio to use the GNOME theme? You can do this (assuming the pre-requisite packages are installed) with something like:
Also, it wasn't quite clear based on the video, but are you using a high DPI monitor?
The last thing I'd ask you to try, if you have the time -- can you try attaching a sampling profiler to RStudio, and trying to see what it's stuck doing during that time? You might have a preferred profiler already, but one way to do this would be something like:
valgrind --tool=callgrind rstudio
Then, attempt to open the 'Save File' dialog a few times, to get the profiler capturing that execution. You could then share the captured profile with us, so we can see what was going on here.