From what i've heard, there's an issue with SR3 and diffuse IBL's image map resolution. This happened to me to. I thought it was an issue with my comp at first, but after some testing, it had to do with the image map resolution in my diffuse IBL when i was using an LDR file (It happens with HDR files too). I had mine at 2800, and it made the material room unusable because of the high lag. So i set it to 100, and the lag went away, and when i was ready to render i set it back to 2800.