Thats because people have low expectations. iOS 11 is very under featured for iPhones, so there really shouldn't be much of speed or responsiveness drop off for iPhones yet there is.
Show me an iPad 9.7 or even Pro 9.7 which doesn't jitter under iOS 11. A lot of people don't notice this stuff. Doesn't mean its not there. Its a joke Apple can't get animations to render properly on processors and GPUs they bang on about being desktop classed. I thought they'd turned a corner with iOS 10 because despite a lot of new iPhone features, it ran very well even on the oldest hardware at release. iOS 11 doesn't even run well on brand new hardware.