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Whatever you do, don't blast it with cold water from a hose or pressure washer. Those "sealed" bearings are not perfect, and when you suddenly cool them off, the air inside the wheel hub or bottom bracket (crank bearings) will contract, sucking water in. This is why you should NEVER ride through a stream deep enough to submerge these parts. You should also never use soap or degreaser near these components, because it can get past the seals and wreck the lubrication inside.
There is no "fast" way to clean a mountain bike that is covered with mud. You must carefully dig out the big chunks, then use wooden implements (I use disposable chopsticks carved to fit cracks and crevasses) to dig deeper, then wipe everything down with paper towels or clean rags that you can wash and re-use.
The drivetrain is a huge issue. I have been trying to figure out the optimum lubrication system for a long time. Unlike with road bikes, you are riding through dirt, which acts like grinding compound when it mixes with your chain lube. The front wheel throws dirt directly on the chain. The only way to really clean it is to remove it, soak it in solvent or "natural" cleaner, then brush it out, then repeat with fresh solvent. I allow the dirt to settle out of my solvent for re-use. Some people use an ultrasonic cleaner with acetone, then heat the chain up in a warm oven to evaporate all traces of solvent before re-lubing. You can clean chainrings and sprockets with an old toothbrush and solvent, but be careful not to get any of that into wheel or BB bearings.
No matter what, if you ride a lot, plan on replacing your chain once a year. As it wears, it lengthens, which wears out your sprockets. Get a Park Tool CC3.2, pic related. As soon as your chain wears 0.5 percent, replace it, which makes your gears last longer. Get a chain that can be taken apart easily with a modern version of the "master link," like a SRAM chain.
