Simplest rule to implement IF you end up doing this...
Idle the engine. That will slow the action of the loader.
As far as slowing the action of the loader, you could use an adjustable restriction as mentioned, or an adjustable flow divider. If you use restrictors you would need two if you want to control both lift and curl functions. Also, since the volume of each side of a hydraulic cylinder is different, the restriction will have a slightly different effect depending on which way the cylinder is moving. Not a big deal, in my opinion. But a flow divider upstream of the loader valve, while requiring more plumbing, also gives more total adjustability and affects all functions simultaneously.
But i would be surprised if the engine at idle wasn't already slow enough. My loaders barely move at idle. Or at least.. that's what it seems like from the driver's seat! I'm sure standing on them is a lot less boring..