Absolutely right! its bad and does not follow “best practice” standards. Just use it if you know what you do and you really have to, as there is no other way to achive the same natively (which normally always is possible)
I actually have never used PostScribe and just one or two times (accidentally) RocketLoader and I am not a big fan of bth.
Does not mean anything as most free opensource JS libraries do have.
If you implement RocketLoader OR PostScribe you do not have to add it as external ressource, but you can save it locally and add it to your combined JS file, so it would make your JS file bigger, but would not produce any additional request. And yes, its clever to avoid external requests.
If you already know/analysed this you already know what the problem is. So the Adsense JS-Part seems not to be very optimized and therefore slows down your site. Does have nothing to do with CloudFlare NOR does it seem that the CloudFlare product (Rocketloader) is not working properly so you most probaly will not get any help here as your problem is 100% not related to CloudFlare.
Can you explain that a bit more in detail? How do you determinate that your site is slow?
Just to say this already: low GPSI ranking does not mean your site is slow! It actually states that it is not well optimized but still can be very fast.