[Coda]

If you're moving stuff across then fragmentation is a non-issue. The move process treats files as singular entities so their organization on the source disk doesn't matter. They'll wind up non-fragmented on the destination drive.

A clean install usually is better, yes. It does require the effort to reconfigure all of your settings, but it also means that you're not moving stuff that's specific to the old computer and it means you have a chance to get rid of stuff that runs in the background that you don't use anymore.

As far as disk fragmentation is concerned...

Moving files doesn't cause fragmentation because the file itself isn't changing places on the disk; it's just rewriting the directory structure to find the same data in a new place. Deleting files leaves gaps but unless it's a bunch of tiny files scattered over the disk (or if the file you deleted was already really fragmented, see below) it generally isn't a problem because new files can just fill in those gaps. The real cause of fragmentation is when files are frequently added to; if there's NOT a gap on the disk for the file to grow into, that file will have to split up so its data is in two or more places on the disk. That's where the performance issues come in -- the spinning disk has to physically move around to get to the other parts of the file. (SSDs don't have that problem, so fragmentation is even less significant on them.)

The huge defrag operations that you used to hear about in the late 90's-early 2000's are counterproductive on modern drives -- pushing all of your free space into one big block doesn't help you at all. Modern defrag mostly just detects files that are split up really bad and rewrites them into the free space on the disk so they're all in one piece. It's a much faster operation that focuses on the important parts.