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:

Chrome CPU throttling


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:

Chrome new CPU throttling

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." :-)