iPhone SE lagging after iOS 15 update

Check device storage:

If you need more space for an update on your iPhone, iPad, or iPod touch

https://support.apple.com/en-us/HT203097

How to check the storage on your iPhone, iPad and iPod touch

https://support.apple.com/en-gb/HT201656

What's the difference between device storage and iCloud storage?

https://support.apple.com/en-us/HT206504

Check and optimize the range of the mobile network.

See this Apple article: If you see No Service or Searching on your iPhone or iPad

https://support.apple.com/en-us/HT204051

See also this Apple article:

If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

https://support.apple.com/en-us/HT204051

Here also use an iPhone SE 2020, have no problems on iOS 15.

I also use an iPhone SE 2020, I experience no problems on iOS 15. Remember your phone accumulates a lot of data over time, so it may be a good idea to restart your phone once in a while. You can also make sure to delete data you do not use. The less memory your phone has, the harder it works to accommodate all the data it constantly sends and receives, the slower your phone will feel.

my iPhone se 2020 started lagging since yesterday after it been power off for 4 hours Music is lagging every is lagging battery health is at 100% I have 16 gb left. I don’t know what wrong with it

Then read the Apple articles in my previous post.