When you use vpn there are two things that can slow you down, first it is the service (i.e. the vpn provider), and secondly it is your machine that is doing the encryption/decryption processing, may it be set top box, pc, smart phone, tablet, router etc.
I have found that with pia (either London or Southampton server) I do not have any meaningful speed degradation for my internet service (fibre, 50mbps down 10 up). This fact is established using my reasonably modest (intel i3 cpu) laptop - modern pc's generally have far more powerful cpu than pretty much any router/stb etc., and because their cpu typically have integrated fpu dedicated for number crunching, they are amongst the speediest vpn clients. At the other end of the spectrum, I am not aware of any router costing under £100 that can run a proper, secure, vpn protocol that can deliver anywhere close to the above speed.
But then of course you don't need 50 mbps to say watch a movie.
If you suffer significant degradation of speed with vpn, to the extent that you are getting well below 50 mbps down consistently with pia in uk using their uk servers, then imho it is the result of the machine (hardware), or potentially firmware, that you have implemented vpn on.