Why my tiller drive super slow

I’m curious why you originally replaced the pump? Was the unit working slowly before, or not at all?

If the unit was already working slowly I would suggest you have some restriction,

On this unit there is a screen and a filter. Check the screen and clean. Check the filter for proper flow.

Check for a suck control valve. These can sometimes feel like they are opening and closing all the way using the lever, but the actual valve inside is not, resulting in a restriction.

Check the ‘dump’ valve. This is a safety pressure relief type of bypass, check to make sure its closing properly.

There is also a bypass which generally works like a regulator on this system. Pressure too high, opens, pressure too low closes. Check that the bypass is working properly

Did you use the same oil viscosity? Its very important in hydraulics. I assume you check the hydraulic oil level…

This is from the owners manual, I suggest you perform this test:

1320/1620 Owner’s-Operator’s Manual 8/30/18 Page 9 of 19
1320H/1620B TILLER TROUBLESHOOTING GUIDE

Remove the tank end (at the T-fitting) of the hose that goes from the pump to the tank. Start the
engine and with the tines and wheels in neutral, pump off about four gallons of hydraulic oil into
a clean bucket. (The .52 CID pump, @ 1000 PSI & 3000 RPM, should pump at the rate of 6.5
GPM.) Replace the hose and remove the tank lid. Block the tines with a short 4 X 4 and stand to
the side.
With tines selected to ON (forward) rotation, squeeze the clutch lever and check to see where the
oil enters the tank. If the pump is good, the oil will enter through the relief valve or the return line
from the filter. If the oil returns only through the filter return, the tine-drive motor is bad.
If the oil returns through the relief valve but does not kill the engine, check the out-put pressure
of the pump. To do this, "T" a 4000 pound gauge into the line at the tank. DO NOT BLOCK
THE LINE OR DEAD-HEAD THE GAUGE INTO THE LINE. With the tines in forward and
blocked, activate the system as before. The relief valve for the 1320H is set at 3000 pounds, and
3500 pounds for the 1620B. If the gauge reaches the specified pressure, the relief valve is OK.
If not, the relief valve is bad and needs to be replaced.
If the pressure reaches 3000 pounds but there is only a small amount of oil returning, the pump is
worn and should be replaced