http://www.peerlessgear.com/node5380.aspx?nid=374646

A few things: Difficult shifting can be caused by damaged shift keys plus sometimes the gears they slide in along with the shaft hardware for that matter. But they can also shift hard from the detent ball being rusted in the bore where it contacts the shift arm. Now sometimes, you will have a neutral safety switch in its place, though probably not on a Murray. That is all taken care externally rather than the shift keys which need to have the box split, provided the shift arm isn't worn. Slow speed if your brake isn't dragging would be an input issue rather than a transmission issue.

Thank you. I appreciate your help. Not sure if the slow speed is caused by an input issue. I rebuilt the engine a couple of years back and put in a new camshaft which greatly improved the performance. 6th Gear used to really fly years ago with this mower. Now it's sluggish at least compared to what it used to be. The brake was barely stopping the mower so I had to work on that. I will check the things that you mentioned. Thank you.

It's possible the brake is adjusted too tight?

Doubtful. The brake was not even stopping the lawn mower. You would press it and I would Coast a very long way before it came to a stop.