Is there a way to throttle javascript performance to simulate a slow client

Under Chrome developer tools -> Timeline you now an option to throttle down the CPU, look for the dropdown:

UPDATE:

Chrome(ium) changed in new versions, it is now under the Performance tab, and you have to click the settings button in the corner for this feature to show up:

This might not be the best solution, but something that could definetely work is to run a virtual machine, there you could specify all hardware specs as long as they are lower than you real machine. Look at this post

I would use a VM and just limit it's resources. If you are not a fan of virtual machines, then I would go find an old machine at a yard sale, thrift store etc. and use that as a testing platform. You can never patch it, fill it up with crappy malware laden programs and then it will be just like the experience for an "average user." :-)