There's definitely not anything in the contract that promises performance to a 3rd party, especially not in a residential contract. The legal options are switch to a different ISP and/or start a new one. Not always easy or practical, but there you go.

Who is to say where the performance problem is? Certainly not your contract.

Maybe if the last mile is cronically congested, or between the local aggregation switch and their regional exchange points, you might have a legal case. But if the issue is insufficient connectivity between their network and other networks, I would be very surprised if the contract terms covered that at all.

There's a bunch of networks throughout the world where their policies mean you can get more economically acheive better connectivity to their customers by hosting outside the geographic boundaries of the network rather than inside it. Doesn't make sense from a theoretical point of view, but when German ISPs won't interconnect within Germany, serve their customers from Poland or France and the connectivity picture may change significantly. Worst case, serve them from the US (but the latency may be too high)