They don’t even have a 6 and haven’t for sometime.Just tested on my old iPhone 6 which is on iOS 12 GM. I force quit all apps first, so none of these are loading from memory.
I used the stopwatch on my iPhone 8. I tapped the app icon and start button at the same time and then tapped stop when the app fully loaded. Obviously timing won’t be perfect, and I’ve rounded tunes up or down a bit for presentation.
Settings = 1 second
iMessage = 1.5 seconds
YouTube = 3 seconds
Uber = 2.75 seconds
I think your iPhone 6 needs its battery replaced or has a fault of some other kind.