As of this moment, my iPhone 11 Pro Max (my current iPhone) is using a profile that started in late 2011 with an iPhone 3GS. It migrated to an iPhone 5, then an iPhone 6+ and to a 6s+. For a year and a half it saw use on a Pixel 3a XL through a transfer and then back to the 11 PM through a transfer. iMessage picked up from mid-2019 where I'd left it.
I've never had any trouble - even during jailbreaking for several years.
But, I don't use my phones as computers and I don't store anything on my devices (long term) that I don't want to lose. I think as well, the fact that I always get the highest capacity Apple offers, allows me to avoid any low storage problems. I don't use iPhoto, all my photos go to the Camera Uploads folder in Dropbox which automatically places them on all my Dropbox equipped Macs and PCs. All my email is IMAP and my contacts are on Google.
Any backups on the device are simply for app and system preferences. In short, as I've said here many times, I could lose all my data on my device tomorrow and it'd either be backed up already or totally unimportant.
As to the specific subject of this thread, both my 11PM, my 6s+ and my 6th Gen iPad are running iOS 15 and I haven't had any issues with slowness or anything else.
My digital life, such as it is, is stored on my Macs - not my iPhones/iPad. And those are backed up nightly to my NAS and weekly to Dropbox as sparse disk images.