could be a partially clogged retract solenoid. these are basically a solenoid operated needle valve, so if the small hole is partly clogged, you will only ger partly functional flow. Only way to check that is take the valve out & check the end where the needle hole is, retract the needle using the 1/4" brass hex on the sire side of solenoid and clear any debris.
Could also be a weak solenoid; I think that would show on the voltage while under operation; You'll either need to make a plug-in-pigtail where you can read voltage while operating the retract, or take readings using an insulation piercing probe while operating the retract.
Please let us know what you figure out. Mine seems to be slow on occasion, not always, but can't remember if its extending or retracting, and its not slow by a lot.
Baja-tested '08 2-slide 36'
Alpine: The Ultimate DIY'er Project