I realize this is an old post but putting this up for any readers that stumble here with the same issue.
Seems this is caused by CPU performance (or lack of it) I noted that my CPU usage for both cores (Task Manager > Performance) jumps to about 65% whenever i make a small change to the Comp. (adding effects etc). It takes easily about 20-25 secs to render. I’m using an AMD X2 250 dual core, with 4GB ddr3 RAM.