One thing that will slowdown the apparent SQL performance is the network connections between the Web server and the SQL server. With MS Access, everything takes place on the local machine and with no external network connections. With SQL, there is a good chance that the Web server is making dozens of network connections to the SQL server for each forum page.
You should compare the IP addresses of the Web Server to the SQL Server. They should at least be on the same subnet. For example, your web server is at IP: 213.202.100.41. If your SQL server is not 213.202.100.x, then the SQL is probably being shared by too many other web sites and the network connection between the servers is too slow. Your host may have multiple SQL servers and you could ask them to move you to a closer SQL server.