From Which? Magazine "How to make your smartphone faster"

Well of course you basically took all your user data off the phone!
I reloaded all the apps and "system data" is now 5GB rather than the 30GB it was before I did the reset. Sorry, it was more like 25GB.

I also deleted about 2GB of photos which are no longer on the device but I have backed up via Google Photos anyway.

It's basically the same phone (a little more responsive) with same apps and a huge amount of local storage restored.
 
Freeing up persistent storage ("flash") isn't doing to speed up a phone's performance to any significant extent. Non persistent/volatile RAM may be a different matter but RAM is also their to be used so just because it is isn't necessarily a problem. A factory reset may well speed a phone up for all sorts of other reasons.
 
I reloaded all the apps and "system data" is now 5GB rather than the 30GB it was before I did the reset. Sorry, it was more like 25GB.
Yes will have been the app cache for YouTube Spotify and the likes, you could just have deleted them and reinstalled them rather than the whole hog but glad it working well again
 
Before undertaking any "space saving" or "speed up" tips, please make a full back up of your phone to an external device, and make sure you know how to do a restore!

Offloading stuff from the phone to the net periodically is also a good idea. e.g. Apple will rent you 2GBs of iCloud storage for €2.99/month. Options like this makes the data concerned "shared" i.e. accessible to your other devices without having to copy files back and forth. There may be a limit to the number of devices that can access one of these shared repositories; Apple's is 5 devices.