After ANY iOS/iPadOS upgrade/update, you should ALWAYS perform a simple hard reset of your Apple mobile device.
For iPad models with a Home button, do a simple hard reset of your iPad by holding down both the Home and sleep/wake/power button simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons.
My 2015/2016 iPad Pro models have been just fine and have NO noticeable slow downs.
ALL of my family’s iPads are still operating/running iOS/iPadOS 14.8, though.
No complaints.
Odd numbered iOS/iPadOS versions simply seem to NOT work as well as the even numbered versions.
iOS 11, iOS/iPadOS 13 were just all real garbage. Especially iOS 11. Total garbage year of an iOS version.
iOS/iPadOS 15 seems to be just continuing to follow this trend of bad OS releases.
I do not like the whole put widgets of any size all over your iPad home screens as this makes iPadOS screen layouts look massively ugly and untidy and there are NOT enough more useful and cooler features for me to even consider upgrading to iPadOS 15.
For me OS upgrades and updates have always been about consistency and stability and not always about the newest, coolest feature set.