Abb robot 1520id slowing down?

Hi everyone and have a nice forum
I have a question about 1520id which is being used in welding process. This robot, with same velocity parameter, is working much more slower than last year. It completed a fixture about 15 min last year but now this increased 25 min. Robot configured %100 production velocity by the way. Is there any idea what caused this stiuation and what we need to do? Thanks for your help.