4k 59.98fps mov file lagging

You have high resolution, high framerate, variable framerate media that's in maybe the worst codec for editing (HEVC). It's likely going to cause problems on a lot of machines.

H264 (AVC) and H264 (HEVC) are compressed media that require more processing power to decode (play) in editing software. HEVC more so than H264.

Since the video is vertical I'm assuming it's shot on a phone, and the framerate 59.975 is not a typical framerate, so both of those things are indications that the clip would have variable framerate, which is like poison for editing software (that likes a constant framerate.)

High resolution and high framerate will just compound the problems.

To learn more about codecs: https://blog.frame.io/2017/02/15/choose-the-right-codec/

To learn more about variable framerate and how to fix it: https://www.reddit.com/r/VideoEditing/wiki/faq/vfr

Edit to add: what I would do if I were you is either:

1. Transcode the clip straight away using Adobe Media Encoder to something like ProRes 422 LT and then import it into Premiere. If you've already got editing done you can replace the clips in the project with the newly transcoded ones. The ProRes will be a larger file size but it's an intraframe codec so easier on your hardware. The resulting file will also be constant framerate. The caveat here is that any issues as a result of the VFR will be burned in, so things like audio going out of sync. If you notice audio going out of sync and need that fixed you'll have to look at my link about VFR to see about correcting it.

2. If the VFR doesn't cause significant problems then you can make proxies into a low bitrate, low res intermediate codec like ProRes Proxy. If you have major VFR issues then you'd notice that the proxies don't perfectly match the original footage. You can do a spot check when you are done editing just to make sure it's not majorly off, or again - see the link about correcting the VFR. I'd probably go this direction because you can always delete the proxies when you are done with the project.

Optional: If the intended output fps is 60fps, then I'd change the sequence FPS to be 60fps (since the clips fps is variable anyway you may as well have it interpolate to what the output will be in the sequence rather than on export.)

Optional 2: It's also worth noting that if you don't have a need to be working with 60fps footage then I'd aim to shoot in 30fps. It's not that common for live action stuff to be 60fps. So is it worth doubling the performance requirements and processing?

Optional 3: 10 Mbps is a pretty low target for 4k 60fps footage. You probably want to bump that up when it comes time for your final export (just in case 10 Mbps was the plan).

2160 x 3840 at 59.98fps. The 60FPS is the gotcha. Have you tried editing at 1/2 reolution or 1/4 reolution? 

Avichai,

Need to know more about the computer you are editing with and the camera that shot this vertical video file.

Thanks,
Kevin