I did an awful job of painting for years, and I think I played less than I otherwise would have like because of it.
Now, when I'm trying to bang out a bunch of mechs, I approach it like this (but I don't know that I would listen to me if I were you):
(0. If metal, superglue the metal mini to a hex, and vinyl spackle on the base around the feet. After it dries, you can use an exacto knife to scratch off stray spackle.)
1. Prime in white if you want light colors like white or yellow or sky blue, and grey if you want something darker like olive drab or navy. Just grab a $4 can of primer at the hardware store. If you want to basecoat other colors, you can save some time by priming in the matte version of the spray primer, but not any with texture (like "ULTRAMATTE", which has a popcorn type gritty texture... I mean, unless you're into that). I use loctite fun-tak for metal ones or double sided tape for plastic to stick them to a piece of 5x8 chunk of cardboard cut from a shipping box for priming and basecoating, and only take them off and stick them to a spray paint can lid when I get to the detailing and basing. I knocked over a lance once and they fell 4 feet to the floor and got all janked up, and since then, I keep them stuck to the cardboard until they are done. Spray in quick bursts from 6-10 inches away. If you prime in grey or black, do a light spray of white from above to bring the details back out so you can see what you're doing later on. I used to think spray primer was not "right" or something, but it works great and saves a ton of time.
2. To base coat, dilute acrylic paints with water until they are a bit runny, a bit like a wash, but you don't see individual pigment specks floating around. You'll do two or three base coats, so it's nice to do one or two lances all at once. I use army painter for tan/green/yellow/brown, some more gimicky colors from games workshop for clan mechs, and basic junk acrylic paints for generic inner sphere. I hate the globby acrylic look though, I would much rather do another coat than have that caked-on thing going.
3. Once basecoated, I use runefang steel on some weapons and actuators, then touch a dot of red or green or blue on lasers or ppc's. For missile racks, you can paint black over it, then gently brush the runefang steel over the top; like very little metallic paint and gentle, light brushes. Now's the time to add some other bits of color or camo, but that's just extra work if you just want mechs to toss on the tabletop. Just keep it simple. I often drybrush a bit of grey or black or a darker shade of the base color to bring out details, but if you're going to do a wash, you can skip that; a bit of drybrush can break up lines and give a worn look, though. Just a bit of runefang steel on the legs and arms can make a huge difference.
4. For cockpits, I just paint them yellow or blue over black, sometimes red; it depends on the color of the mech. I don't really like the jeweling everyone does, personally, but that's obviously taste. I always put a dark wash on cockpits to break up the flat look and add some "noise" to it. It also pools around the edges and gives it more of a "canopy" look.
5. If you want to wash, water down the Vallejo wash a bit or use Nuln oil. You don't need to overdo it, which really makes the mech look gritty. You can just touch the tip of the brush to an area with detailing and the wash will seep into it through capillary action, without necessarily slobbering the stuff all over the mech. I always wash the cockpit and silver bits, but sometimes (because the acrylic basecoat was watery and a lot of pigment got in the cracks, making those spots darker like a contrast paint without a wash) it's not necessary to get a detailed look. The mechs look "newer" without the wash. If the wash is too dark, you can drybrush the color back up, but it sounds like you don't want to waste the time on that. If you use contrast paints, or multiple coats of a watery acrylic for basecoating, the wash isn't always necessary and can sometimes ruin the look you're going for.
6. Basing can feel like a huge waste of time. You can use Agrellan Earth or Astrogranite from GW and call it a day. If you want, use Elmer's Glue to put some sand and green flock on the base instead of the GW stuff. If you want, you can paint or drybrush over Agrellan Earth and Astrogranite, but it looks fine without any extra work. I've used transparent crackle over white to get a snowy/icey effect and it was fun, but this feels like a time killer in general to me. Those Army Painter tufts look kind of cool but I have never used one. You can always just paint it black and say that "it's a battlemech, not a child's diorama", and strike fear into the heart of your opponent.
7. Spray with Krylon matte acrylic to keep the paint from getting easily damaged. The added benefit is that it takes the shine off the mini, so that the color looks better, and the everything just looks much crisper.
That is entirely what I do. I thought about doing paneling and everything, but I don't have the patience. Like, the awesome? Nah, man.
If I'm doing a generic batch of mechs, I usually do a group of six: one light, one assault, two medium, two heavy. Then you can kind of put together a Light/Med/Med/Heavy lance on the light side, or a Med/Heavy/Heavy/Assault lance on the other end of the scale, or whatever. When you're doing six, the first is dry by the time you finish the sixth, so you can just run through the steps with minimal wait time. If I prime in the morning, spackle at lunch, basecoat after work, and finish the rest after dinner, I can seal them the next morning and get stuff done quickly.
Then it's pretty efficient to just bang out a green lance (could be Davion or Capellan) or red (could be Kurita or mercs) or white (could be Comstar or whatever) or grey (GDL or anything). It feels more efficient not to go after specific units, since you can just lump mechs together that don't have that extra level of detailing to make them exactly Lyran Guard, for example. Just do the arms and legs blue and the torso white, and it's in the ballpark of Lyran Guard but could be anything.
I have grown fond of "stippling": you get a small amount of paint on the brush, almost like you're going to drybrush, but then you just rapidly stab the mech with the brush in a small area. The word it makes my mind think is "flecks". It gives a noisy look, and I like the results better for things like camo when I attempt it: just get a small brush, and rotate the mech slowly, dotting out some paths and breaking up the straight lines a bit. If you are conservative about how much paint you use, you can add more flecks by stippling more, and it's a more forgiving way to experiment. If the camo scheme goes wrong, just basecoat over it, or fleck the whole mech to give it a noisy look.
Re: hands cramping up. When I'm detailing or basing, I take the mech and stick it to the cap of a spray can, so I can hold that instead of the base. You can rotate the can upside down to get a better angle. Helps a lot if you're not doing that.
Re: drybrushing. I dab acrylic paint on a crap brush, then wipe it on a cardboard box until I don't see any more paint coming off. Then I use quick strokes over to bring out the detail or make edges look worn.
The wet pallette suggestion is a really good one too. I just put a wet paper towel under parchment paper in a plastic tupperware container.
My mechs do not look awesome but I get them done quickly and have fun with them.
