It sounds like you have one or two problems going on.
First, make sure the WiFi you are accessing through has enough speed for what you are trying to do. If it is a lower speed connection and/or there are multiple devices using it at once, you will have this problem.
Second, you need to clear out your device. Have you cleared out the browsers "Cache" and "History" recently. If you haven't, now is the time to do it. NOTE: Before cleaning them out, make sure you have saved any passwords, log in information, etc., as they can be erased and you don't want to lose them.
Third, do you have a lot of apps running all at the same time? If so, this would cause you grief as well. Make sure you actually "turn off" any apps that could be running in the background. Some apps and games take up huge amounts of space/ram.
Fourth, Clean up what you have on the device. Do you keep a lot of videos, images, and other data on the tablet? If so, it is time to move them over to a place better able to hold all that data. Say a cloud account or a computer, even a thumb drive. This will free up space on the device and also help with your internet issues.