Have you tried a clean install on the 6, and have you checked the battery?You're absolutely right. I was using a 6S till a few weeks ago and now I'm using the 6 till I get a new phone. There is a perceptible lag in the 6, it is downright slow at times and frustrating. But then I remind myself that it is over 5 years old too.
Whereas the 6S was a breeze to use and I hardly ever noticed any lag on it.
An iPhone 6 shouldn't be frustrating to use on iOS 12.4.1, even the 5s isn't. If anything, the Plus model might perform a little worse than the 5s due to its much larger (rendered) resolution, but the regular 6 should be fine.
Of course the 6s performing way better than the 6 is to be expected, but the difference shouldn't be quite like frustrating vs. a breeze.