Choppy/laggy playback, High end PC specs

Did you get a solution for this, i'm currently facing this.

As Bill says, you can't manually change your project settings once your project is underway. You can only set it up at the beginning of your project.

I'm not sure why your project didn't match your MP4 specs automatically when you added your first clip to your timeline, but that's the reason your performance is so poor.

Try starting up a new project, this time manually setting your project to match your video specs. The latter half of this video shows you how.

Premiere Elements 15, Basic Training Part 1 - YouTube

If the GoPro footage is an MOV, the program may not have automatically set up for it. The presence of the "render line" will tell.

Note that the project settings above say it's set up for 1080 30i. If he's loading 1080p60 into his project, the discrepancy is likely what's causing his performance issues.

But once we have his answers, we'll know better.

Try transcoding it with Handbrake before importing into Premiere Elements and see if that helps:

Fixing  video with Handbrake

First, with that computer and 1080p footage, the editing experience with Premiere Elements should be smooth.  I use a 4 year old Asus laptop gamer and even 4K is smooth. 

There is a needle in the haystack somewhere. 

What are your audio hardware settings in preferences?