Pushing the mower was a great test. From your original post, the right side cut well, the left side cut poorly. Correct? This makes sense for a CW cutter.
A few things come to mind. Like the above comment, low tip speed seems very possible. 17 hp on a 52" rough cutter is light anyway... especially on gas powered machine. Diesel is capable of way more torque. Does your mower pull down under load? Do you know the published tip speed? If not, we could calculate if you report the sheave diameters.
If tip speed is sufficient and it's not bogging down, a shallow deck shell would cause a mower to behave this way too. Rotary cutters can process large volumes of grass because they have deep deck shells. Too shallow and they stop processing (or slowing to a crawl like you described.) This is the mower's DNA. Use a tape measure under the mower and determine the depth from the blade cutting plane to the top of the deck chamber. Measure your neighbor's DR too (that cuts well). Much difference?
Hang is there... we'll get this figured out!
