1. check firewall settings (on PC and router)
2. disable antivirus (i had Avira before = masive FPS drops)
3. what is your router/switch you are using, some old devices can do lagg

crossed my mind, I tried it and it did not help

Firewall is off
Antivirus off (ESET smart security)
Router, It is a model "TP-WR340GD"

you have a high ping between the local computers, but not with anyone else right? and when you're playing its not actually playing like its 10,000 ping, but something far less?

...and, if your server is accessible from the world, when you connected to it from home, did you use the public address, or your inside private address?

if thats the case it could be an issue that your client is requesting gamespy ping the private IP address, thus the ping rate is enormous because gamespy can't talk to that address and every ping drops.

(also some routers by default drop incoming icmp messages (sometimes a separate option from the firewall) check that too)

other servers I ping normal
Unfortunately I do not have a private IP :/

Unfortunately, my ISP has blocked me access to manage the router, you do not want customers to have access to the settings ....

I think it would help if I ordered a public IP?
I'll try to ask too for the ICMP messages.

-edit, i misunderstood the issue

i dont think he wants to host it public, he just testing something on LOCAL and ping gets high