I honestly can't think what's causing this, everyone on my forum is complaining and I really need to fix whatever is wrong

Since I upgraded to 1.3.0 everything on the forum is a lot slower... one forum user told me a thread took ten minutes to load, others are saying that the threads take so long to load that they just give up.
The number of members and messages on my board has been increasing at a steady rate ever since I first installed Y1G, so there has been no "sudden surge" in hits etc that might be causing the slow-down.
Has there been any major query changes in 1.3.0 that could be causing it to slow-down like this?
Finally, here's some statistics that I myself have noticed:
Average thread load time with 1.1.0: several seconds
Average thread load time with 1.3.0: thirty+ seconds
One of my moderators who previously praised the forum for being "lightning fast" now thinks it is incredibly slow.
