If I have a phone plan in country X with roaming, what will I get when roaming in country Y?

I find that the level of service is very variable when roaming and I'd be interested to know before my trip if it's worth getting a travel eSIM or roaming on my other contract instead.

Sometimes I find it hard to tell if I'm being throttled, have bad connection, or using a slow network and it's frustrating.

I'd like to be able to enter my home provider & plan, and lookup what I'd get at my destination. Does such a thing exist?