This hasn’t been true for years. Since like iOS 12 or so it tended to be the same or faster. The problem is that the first time it boots after an upgrade things like photo reindexing happen and that didn’t use to give you a warning, so people would say it’s slower and never retract that statement when the problem went away the next day.
Older devices are def slower. I have an iPod Touch 5, iphone 6, iPhone 7, 8 Plus, SE 2020, 13 Pro, iPad 4, iPad Air 3, iPad Air 4. There’s differences with the older devices. They’re clearly slower. Starting with iPhone 8 Plus, it doesn’t matter as much.
How’s the battery condition on those? My iPhone 6 was slower in certain apps but the system performance wasn’t noticeably different.
You can get a second gen SE for like £160 on eBay.