Can you please expand on how much lower speed CPUs and what level upsampling is possible?
Let me give you a few data points that might help:
I've set up my Macbook Pro (2.8Ghz I7 4980HQ) with Windows 10, running HQP at DSD512 with xtr-2s filter and convolution. When playing music, the CPU is pegged at about 40-50%. Not great, but works. Since this heats up the CPU, the fans kick in, and that makes for some noise from the laptop. Otherwise, it functions just fine, the laptop does not overheat. The sound is awesome, and is an obvious improvement over DSD256 (not a huge difference, but obvious).
Prior to doing this, I used the same Macbook pro running HQP on MacOS with DSD256 and convolution (same xtr-2s filter) and it was using up to about 20-30% of the CPU, and almost no noise from the fans at all.