Snapper LT2044 - engine runs fine, but have very little acceleration

[LisaHorn]

I have a Snapper LT2044 and it has gotten progressively worse in terms of acceleration. I went out to mow today and I barely got it back in the garage because it seemed to have no power at all. When I first got it several years ago, it practically flew around the yard, but as the years have gone on, it's gotten slower and slower. I've kept up with oil changes and spark plug changes, etc. Any thoughts on what I should check or try? I did notice today when I was poking around that I can actually move the thing even when the transmission release lever is in the run position.

Any help/ideas would be greatly appreciated. And I'm super hoping no one says "replace the transmission" because that costs half of what the thing cost me in the first place...

Thanks,
Lisa