I agree that BG3 combat can currently feel slow, but this problem shouldn't be exacerbated with an (optional) increased party size:
If anything, without any rebalance involved, playing with a party of six characters currently the fights just go by faster.
You are quicker to make kills, less dependent on the occasional RNG for crowd control, etc.
And most of what makes SOME fights feel slower than they should be is tied to minor annoyances that will hopefully be addressed over time, like the AI "hanging up" while thinking about the next move or minor details that pile up, like the berserk ability being unnecessarily long, dash demanding a confirmation click with included animation of the character powering up (?) on top, etc.
