if its dropping 10ft in 10 seconds you have a serious issue
my guess is since it is what you say a "single cylinder" that it is a multistage telescopic cylinder? or is this just a simplex mast freelift (does not go up very high)

my best guess is you have some packing issues in the cylinder allowing the rod packing to bypass.
The thing about it slowing down when you pull the lift lever is only that it is applying pressure to the cylinder trying to raise it back up. But if it is still falling down while you have that lever pulled it sounds like an internal cylinder issue. (packing).

If when you raise it all the way up and leave it for a while see if it bleeds down then, when raising it up to the top it might be putting enough pressure on the cylinder to hold it there temporarily? or just applying enough pressure on the packing to expand it and then it holds.

There is one test you can do, when you have it raised up pull the return line off and see if there is any drainback. You may get a slight bit but it shouldnt be alot. If you get more than a trickle then you may have a bad control valve or check valve issue. Now when i say return line i DO NOT mean the line going from the control valve to the cylinder, i mean the big hose going from the control valve back to the tank. By all means DO NOT loosen the pressure line while it is up in the air, that is very dangerous and will get you wet with oil in the process. Not to mention the forks will be comming down quickly too!

If your not sure about any of this hire a professional tech to troubleshoot it and repair it.