Thanks for the quick reply! I did as instructed and the playback is basically the same...maybe a 20% improvement, but I'm not even sure.
One thing that gives me pause is when I go to export the files "same as source," it tries to convert them to the codec that the timeline is in, as opposed to exporting them as their native codec. If I do chose DNxHD as the export setting, the render takes a really long time, which makes me think that it's re-compressing all the frames.
Does it seem to you that it might be my system that can't handle the files? That would be very troubling indeed. My 2006 iMac can play 1080p files pretty smoothly so I'm not sure why my PC, which is considerably more powerful, would have trouble with them. My motherboard is almost 6 years old but everything else is fairly new. Do I need to upgrade? More RAM?