I get the same. So much so that i just don't bother playing that many because it is too frustrating. I don't know much about computers, so i am probably not the person to advise you, but a friend had the same problem, bought a new computer with plenty of ram and he now doesn't have the same problem. My C drive is full so that might be my problem, but i wouldn't have a clue anyway.
How many tables you can play does very much depend on your computer as well as the size of your database, and the complexity of your HUD, etc. If you report your problem your logs may give us an indication of how things could be improved.