What is the problem that the downward speed of the hydraulic press brake suddenly slows down during operation?

This phenomenon belongs to “work progress”. It’s normal. There is no problem.
Normal operation of the machine:

After starting the oil pump motor for several minutes, if there is no abnormality, it can be run.

Turn the change-over switch to the “jog” position, adjust the delay time to the minimum, press the foot switch “down”, and the ram will drop rapidly.

When the stop touches the travel switch, due to the action of the time relay, the ram will stop moving down after a certain distance.

If the ram needs to continue to go down, adjust the time relay a little longer, release the foot switch and then press the foot switch “down”.

Press the foot switch “up”, the ram will return, release and stop until reaches the top dead center.