Since the Android 10 upgrade my Fairphone 2 has become so slow it often reboots after struggling to open larger apps like Youtube even after a factory reset. Is there a way I can restore the phone to Android 8 or 9 as Android 10 has made the phone undesirable, if so where can I download the image from?
I would actually recommend to you to re-install Android 10 first as I think there is a good chance your installation had a problem. Here are instructions:
If this does not help, there is also a link within the above instructions where to find older Fairphone OS versions to install the same way. Bear in mind though that when you do a downgrade to Android 9 or Android 7 (Fairphone never been provided Android 8 for the FP2), it inevitably comes with complete loss of data – this should not happen with re-installing Android 10.
Just to be sure: do you use an SD card? If yes, how did you format it? As internal or mobile storage?