When jailbreaking you do have one or two risks to take. After you've jailbroken your device, there is always a small chance Apple may realize you've jailbroken in the past even if you have restored, but that is highly unlikely. I have a jailbroken iPhone 4s myself and so far I haven't had any problems at all. And as for the speed, it is pretty much unnoticeable considering the iPhone 4s has the A5 processor. Another thing, if the device were to completely stop working and you cannot restore, Apple may replace or repair the device. They may go through the device and they will not repair it for you if they find jailbroken files.


As for your other question, you shouldn't worry about corrupt developers as long as you stick to trusted sources.

Just don't download a ton of tweaks and useless stuff from Cydia. I do not think jailbreaking slows down a device unless you download a ton of tweaks and stuff.

You lose you warranty if you jailbreak, but yes it is legal to jailbreak one.