won’t that make every combat take forever though
I generally GM 5e for 7-8 players online, with 1-3 accompanying NPCs.
It does take longer but I find banning multiclassing & feats keeps character complexity down and turns go swiftly. It would be a lot quicker still if the players hadn't objected to side-based initiative & insisted on each having their own discrete turn!
As said above, the main thing is to use groups of monsters. If you are 'building' encounters, do it 4e style, basing it off a standard of 1 monster per PC, not 3e style 1 monster per group.
The best fights IME are probably those with around 1.5 to 2 times as many monsters as PCs.
Increasing the numbers of monsters causes combat time to scale linearly (up to a point, of course). There a lot of obvious GM corner-cutting tricks you can use to mitigate that. Increasing the numbers of players causes combat time to scale exponentially--unless you take steps to stop it.
In 3E, 4E, and 5E (after the first 3E campaign with 9+ players) I didn't give the players a choice. We used some variant of sides initiative or no campaign. This is the trade for running a large group that is going to get through a fight in a reasonable time. Around 6 to 7 players seems to be the cut-off for most experienced GMs. As in, if you were running fine at 4 or 5, you can probably add a couple more without the exponential scaling time getting out of control--if you sit on it, make sure the players are focused, etc. At some point shortly after that, it will spiral out of your control using the standard rules (for a variety of reasons). Since I'm comfortable running on the upper end of the scale and have been practicing tricks for a long time, I could push it to 8, 9, 10 at times, but it was exhausting. Running the way I do now, I can handle 10 players with no problem, up to 12-13 with minimal problems, and handle 15 in a pinch. But I'm pulling every trick imaginable to make that work, and like doing it.
The point being that for every GM, there's a threshold where if you want to exceed your normal comfort zone on party size, you'll need to adapt the rules, and another threshold where some of the options are taken away if you want to make it work. For a GM whose ideal party size is 3 players, those thresholds are going to come a lot sooner.