The iOS version on a device can't be downgraded (Apple doesn't support downgrading), and 9.3.5 is as high as the first gen goes, so you can't install a different version on it.
What have you tried to see if they improve things e.g. have you tried :
- closing apps via the iPad's taskbar
- clearing Safari's history and data : Settings > Safari
- soft-reset/reboot : press and hold both the sleep and home buttons for about 10 to 15 seconds (ignore the red slider), after which the Apple logo should appear
- backing up (and copying any purchases over to your computer's iTunes library via File > Devices > Transfer Purchases) and restoring to the backup
- adjusting settings e.g. turning 'reduce motion' on (Settings > General > Accessibility), turning off 'background app refresh (Settings > General), turning off multitasking gestures (Settings > General)
- resetting back to factory defaults (after copying off purchases and any documents, photos etc that you want to keep) and then resyncing