It could either be a restriction in the hydraulic system or in the electronic control. There is a horse power limiter and boom controller in the platform control box that controls how fast the boom moves. The instructions for how to adjust start on page 156 of the attached manual. There is also a tech tip for "Slow or No functions" that covers testing the hydraulic pressure and inspecting the spool valves. There is one more tech tip that covers adjusting the flow valves. This is for another model but I've had luck performing the same procedure to my Z45/22 for the same issue. All the below documents were downloaded from genielift.com
Down load the service manual and try returning your speeds. There is a process using the toggle switches to speed up/ slow down each function. The boom is gravity down with a straight dump to the tank from the main lift cylinder so a hydraulic restriction is not the most likely issue. The coil/valve use a demand voltage based on the joy stick stroke. Try recalibrating first then go from there