What type of connection you have? Is it possible, that anti-virus softwares will be too sensitive (online scanning), and therefore slows all programs etc ?
Connection >1Gb/s

I'd try to start dedicated server in provider.
I test it also in local network, computer <- -> computer, ping packet size 1500byte <1ms, antivirus software, firewalls ets. not running, when only 2 players - all good, but when 2 players and one ore more bots - greatest lag at the player, on host no lag. Only one player and 19 bots - no problem. I test Maybe i have any problem with hardware

Does my configuration (2Ghz, 512RAM, IDE HDD), is enough to run host game?