First, if there are a lot of devices using the WiFi that can and will slow down the speed for other devices. Also, the time of day can really effect it. Often after 5 pm. You may also want to make sure it is actually the WiFi that the device is using and not data. Sometimes we think it is using one but really using the other. Here are a few other things to try as well..
1. Try and restart the tablet. Sometimes it is that simple.
2. Update the firmware and any apps that need updating on the tablet.
3. If the problem persists, check your time and date on the tablet to make sure it is correct.
4. 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.