It has no place to check fluid. This is one of those plastic transmissions...cheapie I guess.Is it an automatic? If so check the transmission fluid level. Mine got like that and I had to add some AFT to it. It was going slower but also would not pull up a small hill but bringing the fluid back up to where it should be took care of that issue.
It probably runs about 60% speed it did when it was new . That's why I thought maybe the drive belt was stretched or something and put a new one on it. Didn't help of course. There was a transmission recall but I called in the numbers and they told me mine wasn't in that group. Who knows. It's 5 years old this year by the way.It appears to be a sealed unit. Searching google I see that particular mower had issues with its transmission. In fact there was a recall from what I am seeing.
What kind of speed are we talking about? One forum said its top speed was 5.5 mph which is like a fast walk.