Slow rendering only with .mov source files??

Hi, I'm experiencing extremely slow rendering times from short movie clips (2-3 minutes) shot as .mov files on a Nikon DSLR. In-camera setting are at the lowest quality. I've tried converting the files to mp4 and then importing into Premiere with no change. I've done all the standard troubleshooting (cleared media cache, etc) and had same slow rendering on two different iMacs, both with 16GB of RAM. Any ideas on how to get faster renders?? Is there something I should do to the original file that may speed it up? That render timer going up and up and up is killing my projects! Thanks in advance for any help.