Little bit diff specs here, but regardless....

http://www.servodatabase.com/servos/multiplex


Your ques - there's nothing I've seen about trouble shooting a servo to any thing more than what you're already done or to any deeper degree. If its that noticeably slower than a servo its supposed to be faster than, its got probs. These things are pretty complex inside with circuitry and etc.

Its likely you're not going to fix it unless its gear train issue and then its a matter of where to find parts. I'd contact Multiplex and ask about service or just replace it.

http://www.multiplexusa.com/service-support.html