However since it looks browser dependant, it's unlikely to be a bug with the code itself
I'm not good at this, but since on live server I don't have this issue, this means, that something is different here on beta.
Probably, it's some issue with memory-related function (i guess, some time ago there were some works to optimize Elvenar). Tested right now using Task manager. At the moment beta logs out, memory usage jumped from 40% to 100% (16 GB). As soon as I logged out (till server selection page), it dropped below 40%.
Logging out from live server is fast and no changes in memory usage are noticed.