Which type of unlock do you mean?
Most Pixel 2 in the UK will have been sold without a network lock (the biggest high-street chain only sells unlocked phones, even on contract, and of course any bought direct from Google are unlocked). I don't know about the US for this model. But in general network-locked models are slower to get updates apart from Samsung in the US (which must be deliberate, even if their motivation is obscure).
If you mean bootloader unlocked, I don't think the distribution system even knows. It's all on device serial number ranges or something similar. When I've used bootloader unlocked phones with stock firmware I've never noticed any delay myself.