I don't know why you'd need to rebuild your computer every 6 months?
as for the mac, it was noticeably quicker than before, and i explained to my customer that an ssd would be the way to go, but she didn't really want to spent circa 200 quid or more on it, and I have to be asking for that much for the drive and my labour etc.
but as it is, and like i say, it's a good margin quicker than before. Before I clean installed the OS safari et al were excruciatingly slow, I'm talking 20-30 bounces just for safari, but now that's about 3-5 bounces, then if you close the app and reopen, 1 or 2 bounces.
but mac office 2016 doesn't sit very well on it for some reason, excel, word and outlook take about 10-20 bounces even sometimes, but when they are open, the system experiences no slow downs, or crunching, as it was before.
So the clean install helped, but if it were my mac I'd have stuck and SSD in. Easily the best computer upgrade I ever made was buying an SSD, the difference they make is just phenomenal.