Could be the settings on the tablet. It could be the WiFi in the tablet isn't working correctly and needs to be replaced. It could be the ISP limits usage by number of devices or data usage amounts. To many variables.
First check for any updates to the tablet. Both to the OS itself and any apps. After that is done, if it continues to be slow, make sure you turn off 'data' (if it is turned on in settings) and see if that gives you better speeds/connection.
Now if the problem persists, then I would suggest forgetting the connection to that WiFi and turning off the WiFi on the tablet. Next restart (turn off and back on) the tablet. Then turn on the WiFi and try connecting to yours again. If it still gives you the same speed issue, try connecting to a different WiFi and see if you get a better speed. If you do, then the problem is with your WiFi. And that could be a few things. If not, and the other WiFi still gives you bad speeds, then the tablets WiFi may not be functioning correctly. It is also possible that the particular tablet may not accept faster speeds. I don't know that brands tablets very well, and unfortunately most Android devices don't actually tell you what their max speeds are for WiFi.