stick to iOS 11.1.2 for iPhone 8 plus or not?

Youtube is not a reliable source of information unless it is posted by established and reliable testing laboratories, and I have seen no properly done tests that show that any update has slowed down and iPhone 8. Anyway, I don't care whether you agree with me or not. I gave you sound advice. Take it or leave it.

Here's some reliable information: https://benchmarks.ul.com/news/is-it-true-that-iphones-get-slower-over-time

No version is slower according to extensive tests done in the past by several tech publications. Of course, no tests have been done with 11.4 as it is too new, but it's likely none will be because the result of tests with other versions have found that updating has never slowed down any iPhone.