Hi,
I tried what you did, even with high volumetrics and could not find any performance issue.
I tied and switch fro Activity to Tickets and so on for several times, with 10000 tickets and 10000 activities, results are quite good.
Results on Chrome :
Results on Firefox
What you get does not seem to be a performance issue but may be linked to tries to detect loss of connection.
We tried to detect when request to server does not end correctly, due to connection loss, and replay last request when needed. So this may lead to longer response time.
What is sure is that it may not be a ProjeQtOr issue but a network issue : we cannot manage unexpected interuption in network chain.
It seems to occur when some network item (proxy, firewall) unexpectedly ends connection.
Could you check in browser console and search for such issues (connections not ended, reties, ....)
Maybe there is thinks in the retry process that we may improve but it's difficult to investigate as we don't reproduce....