Sounds like you might just be max’ing out the capacity of the coax cable as well (depending on length/signal integrity). E.g. ITGoat (not sure how trustworthy this webpage is, just an example) lists 1 Gbps as the maximum for coax while you would typically expecting less than that, again depending on your situation (cable length, material, etc)

What’s your situation into the wall? Depending on country/ISP/regulations they might give you up to 1000 Mbps under the assumption that it’s a single line going to a single user, however quite often that line is shared with potentially a lot of different customers.

Some countries allows you to buy packages where you have a standalone line going to your wall, however at an additional cost