Honestly, there are only so many things you can do to minimise your assembly time, but it is worth thinking about those things in advance!
First, make sure you have decent, high quality, sharp tools. If you have really good, sharp sprue cutters, you’ll waste almost no time in cleaning up the parts afterwards. If you have old, blunt cutters, you’ll spend more time cleaning up the kit than you do actually cutting the bits off! Or waiting for replacements if you’ve buggered them up.
Using sharp hobby knives and mould line cleaners also help. And …. though it burns my mouth to say so … if you are doing a quick tabletop job with modern quality minis, you don’t even really need to worry about mould lines. At 3 feet distance with modern kits and paint, you won’t see them. For display and close up photos? Oh god yeah, and I hate the sight of them. But honestly, you probably don’t need to bother with the cleverer placement of mould lines on modern plastics, or at least just tackle just the worst examples.
Drilling out gun barrels? A slow precise task that you can replace by dabbing a little blob of black paint on the end of the gun later in the painting process. Again, not quite as good, but more than adequate for tabletop level armies.
How can you speed up assembling a unit? The best trick I’ve come across is the use of the humble egg box (ideally an empty 12 egg box). As you cut parts off the sprues, put the bits for each man into one of the egg bits. It makes such a difference sorting it out and assembling each one rather than trying to cut and glue, or sort through a big mound of parts.
You really should prepare and wash all your models and make sure they are thoroughly free of any oils and release agents, particularly resin. If you are feeling very lazy, you can actually normally get by without this if you have a really good grippy primer, like Halfords grey. If you are using GW or Army Painter primers, you can still get away without it for plastics or metals, but you should still clean resin as it can be quite oily.
Other assembly tricks? Well, if you want to be really lazy, make sure you plan your models out in advance, during non-hobby time. Try to get parts in one consistent material if you want to kit bash stuff – its a lot easier to glue plastics to plastics than resins to metals! Don’t fall for the trap of assembling all the models you have – make sure you’re only assembling the ones you need. If you’ve got a start collecting box but you only want to add the tank from it to your army right now, just build the tank. It saves time right now, and it also reduces the chances that when you add another model from the kit to your army, you won’t have assembled it earlier with the wrong weapon choices.
On the flip side, if you have a planned army, don’t waste time – assemble everything you have planned, so you can minimise the shared priming afterwards! The real key is just don’t get distracted by ad hoc assembly and hobby – build the models you need to build.
These tips won’t massively reduce your assembly time. But reducing it even a little, and actually focusing just on the models you actually need to do makes a difference. If its just enough time to let you quickly undercoat or prime the models this session, so they can dry properly before your next hobby session, even a few minutes can make the difference between wasting a full hobby session with just a few minutes of priming and then having to wait. Its all good, and gets us closer to deploying a painted army onto the field.