You can only fix things inside your house and maybe get your ISP to fix things between your house and them. Even then most ISP don't care about ping time they only would care if you get packet loss.
Since the router and the ISP connection are the same for good and bad application it is not likely either of those.
If you really want to find it try tracert to some of the IP addresses that have issue and compare them to ones that don't. Your goal is find the hop that is causing the problems. You may not be able to do anything about it if it is in another ISP in another country.
Now maybe it is a application problem. Not sure what to suggest bugs in the software could pretty much be anything.
I strongly suspect there is some bottleneck between your ISP and other ISP. ISP in the EU have been concerned about the high usage rates with everyone at home.
>>>kinda a afterthought. It generally does not do much in the EU but there are so called "gaming" vpn that may have a different path in the internet. Primarily they help people in asia where some ISP do not have the best access to undersea fiber. Normally the USA and EU have such fast connections these services only slow things down.