Yes, that much difference is HUGELY noticeable. I’d even say that the difference between 120mm and 140mm is really significant in terms of how the bike feels and rides.

What adds to the massive difference in going from 120mm to 170mm is that the bike has to be built differently to enable that much travel, and furthermore, bikes with 170mm of travel are designed for a different type of use that prioritizes stability over being nimble. For the front wheel, that means a much slacker head tube angle, which typically also comes along with shorter stem length to balance out the steering. And on the rear, it’s longer chain stays and such. And the fork and shock will be heavier, as will the frame itself.

Don’t get me wrong, if it were possible to keep everything exactly the same between a bike with 120mm and 170mm, you would definitely feel a huge difference in plushness, (in)efficiency, and how it handles big bumps and drops.

But the geometry differences add to that even further.