Since upgrading to 26 on my iphone, I haven't noticed any degredation or slowing down.
I was initially reluctant to upgrade, and held off for several months, as I felt that there would be much change for little gain, because I don't use a lot of the capabilities - "liquid glass" seemed like the kind of interface that would consume power for no purpose. I assumed that I would have to waste a lot of time restoring apps to behave as I wanted and already had configured.
However, the reality was much better than expected, and very little input was needed to keep things as I like them. The most noticeable difference for me was with Safari and, having got used to the changes, I think it's better.