Because imovie transcodes your gopro footage to something easier to play, and Premiere is set up to play the footage natively, which takes more computing power.
GoPro pretty much expects you to convert to the Cineform codec for editing. So much so, in fact, they bought the company.
The program is free. I suggest that you take a look at it before doing any more editing of GoPro footage.
Stephen and Steven have given you the answers, and with PrPro, it comes down to hardware, or doing similar to iMovie, and converting the Source Material to something else.
Thanks everyone, very helpful. It's a rabbit hole but I'm starting to get my head around it all. Cheers,