You can’t actually increase the RAM on any smartphone yet (not even FP3), but you can always delete unnecessary files like the cache-memory in your storage to make it faster. When I did this on my Fairphone 2, it was huge (>7GB!) and since then the device is running way smoother. To delete apps you don’t use is a good thing to do as well
Someone also told me once that it’s always good to have at least twice the amount of free space available in your storage than the total capacity of the device’s RAM… So yes, moving files, medias and apps to your micro-SD card should be a good idea I think.
Worst case scenario, you could perform a hard reset, which should solve the problem…