Let’s put it this way. Does IOS 13 run faster with 3GB of RAM instead of 2? The short answer is no. Wil IOS 14 or 15 run slower on 2GB devices? Nobody can tell for sure, but given the recent trend probably not.
The slightly longer answer is, while as fast, apps will refresh much more, which might make you lose unsaved data and takes time, so it’s ultimately somewhat slower, as it takes more to refresh than to just open where you had left it....
Now the even longer answer is, while IOS 13 takes less than 1GB to run it takes quite a bit of that GB. So we have little more than 1GB to run the rest (including IOS apps). How much do apps take? Well it depends, some apps very little, other like Word and other MS Office apps take over 300MB each, so put 2 side by side, open a large file in one of them and you are taking quite a good deal of those 2GB. While you use an app (or a tab), older apps are put put temporarily in a compressed state and will be decompressed to where you had left them (so contrary to windows for instance, RAM is always almost “full” in IOS). IOS compresses better than Android and it can keep quite a few in a small amount of RAM but at some point it will start to eject older apps out of RAM completely (refresh). If the active apps take a lot of the available RAM (see example above) even very recently closed apps will be ejected. IOS will not wait for the 2GB to be completely full and will try to keep some room for active apps to have some room to breathe, but sometimes if the active apps take already quite a lot and one suddenly increases the RAM request (e.g. if you open a large file) it can simply crash the system (and you’ll will be brought to your lock screen). Now add a 3rd GB and you have just basically almost doubled the free RAM for apps. So refreshes will be a fraction of those with 2GB.