I've been experimenting with my routers and UDP/TCP traffic both on my router and Windows 10 Firewall.
I noticed that the problem might be on Blizzard's end as they couldn't be bothered to provide much less servers for Diablo 1. Other times I can log in flawlessly, sometimes Latency skyrockets. It could be that only available region is global when it comes to B.net 1.0.

Is there any way to improve performance and connection from user side?

Far as I can see based on what I've tried tonight, no. You'll need to just direct connect to people for now.