A few suggestions and comments:
1) The slowness you describe is not UFS 2.1 vs UFS 3.1, it sounds much, much slower than that.
2) Since you have looked in developer options already, check the "background process limit" section in there. The best option is "standard limit", which should be the default. Unfortunately you can't control what "standard limit" is, and it will be whatever OnePlus have chosen. You could try one of the other options, but on my Galaxy s21 the highest is "at most 4 processes", which seems pretty low to me. But if that works better than "standard limit" the problem will be that OnePlus have set the limit even lower.
3) While you are there, make sure "don't keep activities" is not turned on
4) I generally advise people not to worry about RAM. 8GB is plenty for running android 13. 4GB free bothers me more, but may be related to your suspicions about background processes. Generally you don't want a lot of free RAM, but occasionally a manufacturer misconfigures things (OnePlus themselves have done this before). You presumably haven't installed any "system optimising" apps (task killers, "RAM boosters", or any of the other stupid names they give those things)? If so, try removing them.
5) Probably not related, but I would get rid of ES Explorer. There have been major privacy issues with that app, and I would not allow anything from that developer on my phone. It was actually removed from the Google Play Store for fraudulent activity (ad click fraud), which is a clear sign that the developer has no scruples and so why would you trust them with access to your files?
6) Just to make sure the problem isn't some app you have installed, try rebooting in "safe mode" (will require rebooting while pressing some buttons, which ones depends on the phone. I don't know this model, but I'm sure there are instructions on the web). If you still have problems in safe mode then the cause is in either the way OnePlus have configured the system or in one of the apps they pre-installed. If it works OK in safe mode then one of the apps you have installed is the problem, and you "just" need to work out which one.
If nothing fixes it, maybe complain to OnePlus because if the system is badly configured that is their mistake. I have never seen a Nord in real life, never mind used one, so I don't know whether your experience is normal for this phone or not (but it sounds so bad it's hard for me to believe this is normal).
Good luck!
1) The slowness you describe is not UFS 2.1 vs UFS 3.1, it sounds much, much slower than that.
2) Since you have looked in developer options already, check the "background process limit" section in there. The best option is "standard limit", which should be the default. Unfortunately you can't control what "standard limit" is, and it will be whatever OnePlus have chosen. You could try one of the other options, but on my Galaxy s21 the highest is "at most 4 processes", which seems pretty low to me. But if that works better than "standard limit" the problem will be that OnePlus have set the limit even lower.
3) While you are there, make sure "don't keep activities" is not turned on
4) I generally advise people not to worry about RAM. 8GB is plenty for running android 13. 4GB free bothers me more, but may be related to your suspicions about background processes. Generally you don't want a lot of free RAM, but occasionally a manufacturer misconfigures things (OnePlus themselves have done this before). You presumably haven't installed any "system optimising" apps (task killers, "RAM boosters", or any of the other stupid names they give those things)? If so, try removing them.
5) Probably not related, but I would get rid of ES Explorer. There have been major privacy issues with that app, and I would not allow anything from that developer on my phone. It was actually removed from the Google Play Store for fraudulent activity (ad click fraud), which is a clear sign that the developer has no scruples and so why would you trust them with access to your files?
6) Just to make sure the problem isn't some app you have installed, try rebooting in "safe mode" (will require rebooting while pressing some buttons, which ones depends on the phone. I don't know this model, but I'm sure there are instructions on the web). If you still have problems in safe mode then the cause is in either the way OnePlus have configured the system or in one of the apps they pre-installed. If it works OK in safe mode then one of the apps you have installed is the problem, and you "just" need to work out which one.
If nothing fixes it, maybe complain to OnePlus because if the system is badly configured that is their mistake. I have never seen a Nord in real life, never mind used one, so I don't know whether your experience is normal for this phone or not (but it sounds so bad it's hard for me to believe this is normal).
Good luck!