Does jailbreaking an iPhone have an impact on performance? I'm using iPhone 6s plus 64Gb with 9.3.3. Does jailbreaking have an effect on performance/speed?
No. Jailbreaking itself doesn't cause slowness. It's what you do with that that causes trouble. And since you have a 6s.. I wouldn't bother jailbreaking it until iOS 10, since it's so close.