1. I would suggest you clear out the cache of many of your apps. The ones that save a lot of data to the tablet. As long as it isn't data you really need, there is no reason to have it bogging down the device.
2. Next try removing apps you don't use but are still running on the tablet. Many apps run even when you aren't using them and that takes up resources. If it is an app you really never use, install it. For those you do use, when you are done, make sure they actually shut down and aren't running in the background
3. Try and restart the tablet. Sometimes it is that simple.
4. Update the firmware and any apps that need updating on the tablet.
5. If the problem persists, check your time and date on the tablet to make sure it is correct.
6. Should none of the former work...
a. Clear out the WiFi data on the tablet.
b. Reboot the modem/router.
c. Once the modem/router is fully 'up' again, reboot the tablet and re-input the WiFi data.