To answers some of your questions server performance suffers before during and after server updates because we do rolling updates so we don't shut down and update all of the servers all at the same time. So while some servers are active others are updating putting more load on those that are still active during the process.
Also the reason we add new waves of players to the game is to stress test the servers and see what we can do to either optimize to avoid the problems caused or add servers and find where are more servers needed.
The hours you speak of poor performance are 'peak' hours when most people get off work and sprint home to play Escape from Tarkov.
I hope this helps!