Sorry to hear that it was a bit disappointing.
There's a tip in the main rules that when you've got a character (especially the Killer) with Carnival of Carnage, you often need to up the mook count, otherwise they will very easily clear the field of mooks and then get started on the Foes - and the Killer is good at downing those, too - leaving other PCs with little to do. (I had a Killer whose Both Guns Blazing attack roll exploded ridiculously, and they one-shotted a Featured Foe demon as it burst from the stagnant flooded cellar - it was certainly dramatic, though!) I tend to use the guideline from the book of three mooks per PC, with an extra six for mook-sweeping PCs like the Killer - Red Packet Rumble is set up for shorter fights, as a demo, which is also why there are fewer Foes than PCs as well as fewer mooks than usual.
As in some other games (early editions of Shadowrun, for example), characters with very disparate Speeds can really make the initiative gap noticeable with different rolls, though, you're right. Targetting the 'fast' characters with Foe attacks can force them to spend shots Dodging, though, which can slow them down. (And is genre-justifiable as, at that point, they're the obvious threat to the bad guys ... having someone frantically shout "Kill them!" is always stylistically appropriate.)
Also, the characters with lower Speed often have other useful abilities, but those don't always come through in a short session, especially one that's not necessarily tailored to the PCs in use (always a challenge with pre-written adventures). In the longer-run, the Initiative roll luck will tend to even out, but as a first experience, especially with shorter fights, I can see that it could be disheartening, especially if the player is used to "each person goes in turn" mechanics like D&D rather than the variable flow of FS.
I've not run Red Packet Rumble, but having two Supporting Character GMCs in the opening fight does make life slightly more challenging for the GM, too. Having fewer Foes than PCs also gives a sort of screen-time advantage for the high-Speed characters in the fight.
(Also, Red Packet Rumble hasn't been updated to Second Printing errata yet, which change the rules for Cheesing It and for actions taken in the last three shots of a sequence.)
I'll admit that I regularly use a scrap-paper scrawl-sheet for doing the damage calculations in my ongoing campaign (and, when I had an Old Master doing the Spinning Kick Of Way Too Much Damage, used an actual spreadsheet). Playing on Discord has helped, too, as the dice roller streamlines the calculations, and the players just get their result, rather than having to do the sums. But yes, if you're actually rolling physical dice (which I do for the bad guys), then being able to do the swerve quickly is essential, and can start to get unexpectedly tiring.
I've had to remind my players sometimes that the actions are best planned as ...
- State who you're attacking, not the descriptive details of how (other than whether shooting or punching and anything that affects the difficulty).
- Roll dem bones.
- Describe how you did (or didn't) acheive the outcome the dice have dictated after the roll.
I've also found that people are often more descriptive when describing martial arts moves than gunfights, as "I shoot the two mooks" becomes common. Encouraging "I shoot at them while ..." as a descriptor helped, as it encourages things like "I shoot at two mooks while diving over the bar". Also, players should be encouraged to describe how they Dodge - what are they ducking behind? I've also found that, as in Red Packet Rumble, having a bunch of prepared descriptions for what the mooks or Foes do can help, as that kind of description from the GM can encourage the players (or be used as suggestions when player imagination is flagging). (I also often watch an action film a little while before running a session, to get some more ideas for how to destroy locations and have mooks go down in dramatic ways.)
Hope that this hasn't put your players off too much. The mechanic and flow is different from a number of other games, so that can sometimes be a challenge, and doesn't work for everyone, but it can also depend on the combination of characters in use, and the way the adventure is tailored to them. Glad that you mostly had fun, though.