Try resetting your tablet and loading only the apps you use. Slowing down as a device ages is often to do with apps doing useless things, like loading ads and spying on you, in the background. This gets worse as app publishers update their apps to do ever more intrusive things in pursuit of revenue.

If that doesn't improve your life, check if there is an aftermarket distribution, like Cyanogen, that runs on your tablet.

EDIT: You may also be able to disable built-in apps you do not use, even if you can't uninstall them.