New user, find that video quality is terrible and the program stalls a lot!

Hi there, so the first thing I want to disclose is that I'm a complete amateur when it comes to video editing.

So I wanted to ask.  I took some raw footage on my iphone (.MOV files).  I added about 18 short separate files (about 35Mb - 65Mb per file -- or 30 - 45 seconds per file), which I was to trim down in the program.  Anyway, I noticed that the images on the preview were very grainy.  Like the program had compressed the files and lost a lot (a LOT) of information.  I figured, it was just the display, and when I export the final project it would be ok.  However when I exported the file, the video quality was better than on the preview screen, but it was significantly lower than the original files. 

I equate it to buying a 4k BluRay movie but only able to watch it on my parents old CRT Tv with the twisty knobs, that's how much the difference in quality was from the raw footage to the finished video.

The second issue I had was that the program was stalling a lot, especially later in the edit.  When I was trimming videos (Smart Trim), there's a button on the bottom right that says "continue editing...", that button stopped working and I had to "x" out of the trim and was asked to save changes.  Also, the videos themselve became very corrupted on the preview screen (later in the edit) where the video wasn't even showing and it was all colour bars and static.  I had to "render" the file for it to be ok.  

I'm just really disappointed, that's all.  Being a paid software is one thing, being a product from a company like Adobe is another.  I'm hoping there's a reasonable explanation to this?