Using mogrt files really slow at previewing and encoding

Mogrts are usually slow in Premiere Pro. Rendering them (Pressing enter with the timeline selected) will make them play better but you have to wait for the render to complete first. If your MOGRT is modified the way you like it you can select it then go to the top of the interface and choose clip/render and replace and convert it to a movie which will play back better in your timeline.

Never used any of the Envato mogrts. But as this is a rather common issue with them, they've either not been well-designed for performance or simply have some cool but heavy-lifting bits.

You might import one into AfterEffects, and see what all is going on there. That does look like there's several graphics things moving at once.

Neil