Long SSH/SFTP/applications connections on Ubuntu

Hi,
since I've bought a new server from game dedicated servers offer I have the same problem. Since my first logging after installation 4/5 SSH connections are aborted after a few seconds of not responding, but if I'll succeed it works fine. SFTP is the same (but sometimes listing of folders lasts that long i get dosconnected). I installed there LAMP with phpmyadmin and I cant connect to it by website, because im connection so long. But the worst problem is that my aplication (which I migrated from second machine is was earlier and there was working fine) is really lagging, because it waits really long for retrieving external addons api. Also users, who are loggining in take much time to do it, but after connection is established everything works fine.

Im using Ubuntu 18.04 LTS and the most important is that problem persist, since fresh installation. Rebooting to update kernel didnt do anything. My IPtables are all flushed, ufw is inactive.

I've disabled all game offer firewalls in ovh panel. I also compared it with my second machine and it doesnt differ. The only things that are new are IPMI: remote KVM and SOL oprions, that ive restarted only once.

Does anybody have any idea, what i can do?
And yes, i've contacted support, but you know how long it takes to get answer, so im trying to fix it myself, because my users are getting annoyed ;/