Video is not fluid

What is your video source? Premiere Pro is currently not designed to work with variable frame rate video such as video that come from an iPhone, etc. - without conversion first.

MtD

Because iMovie is a much simpler program, designed to work on amateur and moderately powered gear. PrPro is somewhat of a heavy-weight beast, with some amazing capabilities. And requires a ton from the hardware, and some knowledge from the operator, to get the best use of it.

Now, if you can give some info as to the specs of your computer, the OS/CPU/RAM/GPU/vRAM, the number and types of drives in use, the complete numbered version of  PrPro, the type and nature of the media you're working with (codec, frame-size/rate, camera or screen-capture or whatever), we can probably give you some very good ideas as to how to work best.

There's a few easy things to throw out ... such as say, making sure the previews for the sequence are in an intraframe codec like Cineform, DNxHD/R, or ProRes, and when you're in a place where the warning bar above a sequence is yellow or red instead of green, you can hit the "enter" key and PrPro will take a bit of time to make previews with all edits/effects burned in so that it can play back smoothly.

You can have PrPro make proxies if say you're working with 4k especially if it's an H.264 codec or drone footage, both of which are highly compressed inter-frame and therefore stress the heck out of the CPU.

But give us some ideas what you're working with, we can give specific ideas to make it work better.

Neil