Everyone knows about that tactic with 1-battalion divisions. Rather than training divisions normally, you could recruit 1-battalion divisions, instantly deploy them, then swap template to actually needed one, and then send those proper, but inexperienced divisions to training - that combination is faster than normal training, and also gives you Army Experience.

But, i come up with new idea. Instead of 1 battalion of infantry, you should use 1 battalion of towed heavy weapons: artillery, AT guns, AA guns, rocket artillery. Towed field artillery is the cheapest and earliest from them all, so let's go with it.

The reason for this is because fielding artillery 1-battalion divs is fater than 1-battalion infantry. Yes, single artillery battalion is somewhat slower to train (120 training artillery, 90 infantry), but artillery have 500 manpower, while infantry has 1000. That means what you can train 2 artillery reserve divisions in place of 1 infantry reserve, therefore effectively being able to train them faster.

For "spherical cow in vacuum", but mathematically correct example: let's say we want to train 100 divs, and we can train 10000 manpower simultaneously. For that, Artillery one battalion divisions need 5 "shipments" (with 20 divs in each), while Infantry needs 10 "shipments" (10 divs in each). Put in division training time and you can see, what to complete this task, Artillery Reserve Divisions need 600 time units total, while Infantry Reserve Divisions will need 900. In practice, Artillery will recruit even faster, as limit of simultaneously trained manpower is increased by increasing numbers of actually fielded troops.

There is one thing even faster than this: Penal Battalion. They need 850 manpower and 50 training, so they can do the same "10000 limit, 100 divisions" task in 425 time units (11,76471 divisions in each "shipment", 8.5 total shipments). The problem is, they can only be Unlocked via the Soviet Union’s Penal Battalions national focus; if you haven't done it yet, or you are not Soviet Union, you will stick with artillery.

It seems like no one cares? Unlike the "combat worthy 1-battalion divisions" (what is arguably nonsense), the 1-battalion artillery divisions look useful in practice - on paper, Artillery Reserve Divisions are better than Infantry Reserve Divisions, and Penal Battalion Reserve Division is even better. Did you find any flaws in that strategy?

I found one flaw in that strategy: production costs. Infantry battalion needs 100 infantry equipment while Artillery battalion needs artillery. That means what even battalion with T3 small arms (69 production) costs more than battalion of T1 cannons (126).

But remember: reserve divisions don't and shouldn't go into battle, so they could be fielded in the rear bare-handed, and then their template changed to something useful and what has needed equipment in storage (after that they will re-stock).

Any other flaws?

generally people are more active in the discord. generally your idea is useful, but the extra time to setup 5 army xp, and needing artillery instead of guns to have the base equiptment makes it harder to setup.

Well, extra 5 army XP cost is the same, both in infantry and artillery one-battalion divisions. But in the long run, lots of divisions training in the field after deployment will give a lot more than 5 XP, so it will pay off quickly. So while this is useful in normal gameplay or in multiplayer, it's less useful in speedruns.

Artillery instead of guns can be ignored if artillery reserve divisions are fielded while being under-equipped (they will re-stock with new gear after template change). Besides, if your army uses artillery in their proper templates (support or normal), then your divisions are going to use artillery anyway (and if you are planning to use artillery in divisions, you are going to produce enough of it for whole army anyway), so artillery given to reserve divisions will be then used by real divisions after template change. You can also give reserve divisions low priority, so they won't "steal" equipment from real divisions (though that may result in proper divisions being fielded while wielding some outdated junk, if you do have outdated junk at storage).

If i took it wrong, do division need some minimal amount of equipment to be fielded (so can you field them bare-handed to instantly change template)?

Increased speed on which divisions can be trained and fielded are not only benefits of reserve divisions. For example, amount of special forces divisions and amount of volunteer divisions depend on number of divisions without regard of what templates of those divisions; fielding lots of reserve divisions can increase these parameters. The same thing goes to amount of divisions on border with country what has non-aggression pact, what would allow to prolong it (though this variant has risk of reserve divisions being dragged into fight). And such high number of divisions would confuse opponents. Not many of these are used in speedruns, but it's great in normal gameplay and in multiplayer, to the point when one-battalion divisions are banned on some multiplayer servers.

Generally, while reserve divisions in general are good in play, but less useful in speedruns specifically, what do you think is better in comparison to each other in reserve role - one artillery battalion divisions or one battalion infantry divisions?

https://hoi4.paradoxwikis.com/Land_units

It seems, in new meta, 1-battalion Irregular Infantry is better for this quick recruiting trick. It has 30 training and 1000 men - it's entire point being weak and quickly assembled.

Let's say we want to train 100 divs, and we can train 10000 manpower simultaneously. For that, Artillery one battalion divisions need 5 "shipments" (with 20 divs in each), while Irregular Infantry needs 10 "shipments" (10 divs in each). Artillery reserve divisions would complete that in 600 time units; Irregular Infantry would complete that in 300 time units.

Irregular Infantry is not only faster, but also requires 80 Infantry Equipment instead of 100. That also makes it better for being 1-battalion reserve divisions.

So, Irregular Infantry reserve divisions are the best for forced division creation via template changes.