snapdragon 810 had issues with heating(thermal throttling ) especially when you were playing graphic intensive gaming or 4k recording but still heat was tolerable at least for me... rest top end snapdragon chip fixed it . It was this bad reputation of SD810 that many manufacture went for slower Snapdragon 808
I had x compact, bought instead of z5 because far fewer issues, z5 chipset just too hot and unreliable. On issue with x was Sony lost its way and gave mid range spec on compact phone like other manufactures previously
The people who buy the compacts of this world, are buying a compact because they want a compact phone, they aren't buying it, only for someone to turn around and say, "hey but you can get a much bigger phone for £8 more"
Actually the faster CPU (former flagship SD810) in the Z5 seems to be of more benefit all things equal but it doesn’t make a massive difference. When I got rid of my Z5c it was on 7.1.1 I think and it hadn’t slowed down, it was just the AR coating on the camera was a bit scuffed and camera was slower to use and focus than I would like. I had an XZ that I upgraded to Oreo which broke it - outrageous lag (2-3 seconds to open the keyboard) and the phone got blocked so I got rid. And now I have an XZ1 Compact that had Oreo preloaded - it’s super quick and lag-free but it does have 4GB RAM and an SD835 so it’s right at the top. Be ware of updating your phone if you don’t need to is all I’ll say and if you get it from CeX online and ever need to return it, do it in-store, they tend to take it back no fuss.