I had this problem recently, but I managed to fix it for now and get back to 60k/xp per hour. I did this by downgrading the client back to v1.40. I downloaded v1.40 from
here, and then I opened it using winrar and replaced the version.dat located in the resource folder, with the version.dat from 1.42. When you start up 1.40, it will now be called 1.42 and the mouse speed is faster, meaning that you can get more xp per hour.
The problem is caused by the new version of the client having a slower mouse speed, it affects a lot of scripts. I heard the problem is going to be solved in the next release.