Loss of smoothness as soon as I drop 4k 60fps footage onto Premiere Elements 2020 timeline

Hi,

I hope you can help me please.

After upgrading the ios on my Macbook, I also then had to update from an old version of Premiere Pro, which had become unsupported. Not wanting to spend loads of money just to edit occasional holiday videos, I went for Premiere Elements 2020. Since doing so, I have struggled to find a way of editing 4k 60fps footage from both my GoPro 10 and my iPhone X without degradation of the fps resulting in a loss of smoothness of the footage.  It seems that the raw footage itself is perfectly smooth when viewed in QuickTime Player and also after converting it using Handbrake (in a format that maintains 60fps), which it seems I have to do just to be able to import the footage into Premier Elements 2020. It even seems fine when viewed from the Project Assets window after importing it. However, as soon as I drop it onto my timeline and view it in the main window, it degrades to a very unsmooth state, certainly significantly less than 60 fps, despite me selecting a project setting of AVCHD 1080p 50fps (the highest fps I could find) and also having rendered the timeline. Having invested in the latest GoPro so I could get the best quality possible when watching back on YouTube on a large TV, I'm finding it really painful watching any moving footage, especially something like skiing where reasonably high fps is crucial. 

Am I using an incorrect project setting? I must be doing something wrong, as I surely shouldn't have to accept that this is the best quality I can get from Premiere Elements 2020. Please help!