It is also slowed because there is very little fully open source phone hardware, today - and of what there is, very little of it is pocket sized.
What I don't understand is why, if Pixels can be unlocked and locked back up again - which is part of why Graphene only works on Pixels - why don't more alternative OSs target Pixels? Ubuntu Touch, for example, only supports the 3a. I have a 9 so I can run Graphene, but I'd love to be able to put some flavour of Linux on it.