How to downgrade iOS from 26 to 17 or 18

You cannot go back, so no reason to continue asking.

That "lagging" is a result of an indexing process that must take a look at all of the data on the phone. This will normally take 3-4 days. You may notice that the phone is running warmer as well.

Things should be back to normal next week.

You can't. Apple stopped signing them for iPhones capable of 26.

(262610)

There are no options to downgrade. Apple has never supported downgrading.

What you can do is tell us why you want to downgrade......even though it is not possible.....so we might be able to help.

Your tag line suggests you have an iPhone 13. iPhone 13 was released in 2021, so at this point, lagging on your phone could be caused by:

1. An aging battery
2. Lack of Memory

Open Settings > Battery > Battery Health - what is the health of the battery?

Open Settings > General > iPhone Storage - how much memory is available?