You've made two key changes:
- Allowing multiple combat half-actions per round
- allowing movement to be replaced by a non-movement half-action
The primary factors I see are:
- increase in Psyker offensive capability (Most of their offense is more powerful than their allowed weapons; it gets worse under RT or DW, due to a different psychic system)
- decreased access to defenses¹
- decreased emphasis on movement²
- increased emphasis on non-action defense (IE, cover)³
- breaks the equipment bonuses
- decreases incentives to use burst and full-auto autofire modes
- cybernetics and talents with extra half-actions become devalued⁴
- initiative becomes more important⁵
- Ganging up becomes less essential⁶
¹: Since one gets very few defenses per turn, this change of yours makes them less valuable, by allowing them to be overwhelmed faster.
²: The current system makes movement a valuable and essentially irreplaceable part of the round; take it or not, it's lost if unused or unusable. It's not terribly realistic, but is very cinematic. And DH, RT, and DW are all intended to be very cinematic in tone.
³: At present, especially for melee, active defenses (dodge and parry) are quite valuable; cover is less so, but not enough to render it useless. When one allows replaceing movement with a second attack, cover suddenly becomes MUCH more attractive, as it's an assured penalty to be hit, instead of a roll by the defender... but it also reduces drama.
⁴: At present, the only way to get a second attack is expensive talents and/or cyberware with inexpensive talents. If a character can just hole up in cover and double shoot, these talents are far less valuable, making the Assassin and Guardsman's additional combat actions less useful.
⁵: Further, by allowing the etra attacks, you can run a target out of actions sooner, as well, so initiative becomes more important to prevent having actions drained in defenses.
⁶: Ganging up is the great equalizer in RT and DH combats; it's how one overwhelms defenses. Given that every character can defend once for free, and once by aborting, if you can attack twice, you've just eaten both defenses this round, meaning you only need one attack from a buddy to get through. The norm is that you need two buddies to negate the defenses, or one buddy with a multiple attack talent.
It sounds like you made these changes with rank 1-2 characters... it's going to be more profoundly off-norm or higher rank characters, as normal slow growth of actions is made to feel even slower by providing far less of a bonus over not having them.
SlyFlourish, producer of several books and tips for DM, noted that at level 1 it was a deadly grind.
It might be intentional but I didn't like it that way so I've up-leveled or made them level 2 very quickly.
It was a grind at level 2 but a GOOD grind. At level 1 it's deadly.
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With that in mind, consider leveling them up right away, and that should help many of the issues. At the very least, giving each character a significant percentage of their hitpoints by leveling should help them survive more.
You can see more of SlyFlourish's suggestions for a smooth run of the Greenest in Flames section here
Best Answer
This question is difficult because it greatly depends on an understanding of a specific party's composition, prior healing habits, and available resources. However, some general points can still be made.
Efficient healing does not significantly speed up or slow down combat
Faster healing does not mean more healing
Just because characters would be able to use healing and still have their action available does not mean that they will heal more. Generally, characters will heal only if they need to be (eg at risk of going unconscious). This change from standard action potion-drinking to bonus action potion drinking will probably not change this. If a character needs healing, they will drink a potion or get help from another character regardless of action economy.
Cleric, Bard, and Druid already have access to bonus action healing
If the party already has access to healing word, then they can already do bonus action healing, and having potions as a bonus action would only offer slightly more access to the ability.
Healing is limited more by resource than by action economy
Even if you make potion drinking a bonus action, the party/enemy is still going to be limited by the amount of potions they have. Unless they have an enormous amount, an efficiency gains that might be present from bonus action potions will not have a chance to have a significant effect.
And even if more efficient potion-drinking did cause a party to heal more often, they would just run out of healing resources much more quickly.
Access to healing is a much bigger factor than the improved healing efficiency in combat length
A party or BBEG with access to healing is going to be able to stay on their feet longer than a party without in general. Take away healing, and the battles probably get much shorter. Give your BBEG 20 potions of greater healing? The battle is going to take much longer regardless of the action economy of using them.
In the end, potion drinking efficiency has very little effect on combat times.
Of course, that does not mean that it is a good idea to adopt this houserule, but that is another discussion entirely.