This is not quantum physics. The slide should be loose enough to move easily but not so loose it would slide out when you are using a plunger mute and need to play any valve combination that requires the use of the third valve, like an Ab. Can anyone here actually hold the 3rd slide finger ring and a plunger mute with your left hand at the same time? Didn't think so.
Unless, of course, you actually want to play a 1/2 step flat on that particular note.
If the horn sits on a stand and the slide immediately slides to the stop, it is slightly too free and should be greased/lubed/oiled so it moves only a little or not at all. Using a plunger with this type of slide movement would be a very iffy event.
This also has nothing to do with compression in the valve or a water key.
There are many products actually designed for slides which can be bought in most music stores or on line. These products, when used properly, will not gum up your valves like products that are designed for an entirely different usage.
This IS very easy. Find the correct grease, oils, or combination to get it just right. I would suspect that requirements for each trumpet could be different. Experiment until you find what works. Personally, I use grease, then a slight bit of oil to get the action I like.