If you want to revert the firmware to factory, you will need to do the true factory reset. If you type devmodeon in the search box and search, you will find a Developer options setting at the end of the More => Device information. The factory reset from that menu will revert your firmware. Or you can do the
Manual reset your Kobo Aura H2O procedure.
Basically, the procedure has you restarting your Kobo H2O 5 times without allowing the boot process to finish which triggers the factory reset.
As for which firmware to run? I've always stuck with using the latest firmware and haven't noticed the slowdown you are mentioning so can't offer much help there. If you want to run a downgrade firmware, you can download older versions from Patrick Gaskins'
Kobo Firmware Downloads site. The download links there go directly back to Kobo's servers.
Please note that going back to earlier firmware versions is done at the risk of running into incompatibility with database updates. I would recommend a factory reset before downgrading.