Check that the windows firewall is not intervening the connection or the firewall of the router.
Once that is reviewed, check that you have the UPnP option enabled on your router.
If the same thing continues, I advise you to change the NAT type of your router.
Test
SELECT pSkin FROM users WHERE username = ?
It would stay that way, I think:
local skinID = exports.fs_sql:_QuerySingle("SELECT pSkin FROM users WHERE username = ?", username)
Have you already set the server fps limit to 100?
SetFPSLimit
It can also be poor optimization of resources, textures, models, etc.
As for the server, you can try increasing the priority of the Mtasa process to high to force more usage of cpu and other resources.
Probably have scripts that throw errors so there are a lot of junk logs. Try to fix the scripts, you can temporarily delete the log file from time to time.
Is your internet connection stable? It could also be due to the firewall of your PC or Router, make sure that the ports are correctly open without restrictions.