I play online almost exclusively these days, using Mote. I DM two campaigns, and this particular issue comes up often enough.
First, you have to keep things interesting. Try to design combats with more than just "attack roll -> damage roll". It's not a simple task, but it's really important to make combats interesting. Add some ranged enemies, healing enemies, mages, etc. Throw in environmental hazards, or effects which occur between turns. Give the PCs a time crunch to win. Give them a reason to be concerned.
In terms of process, I have found that the method of forfeiting turns does help. If they take too long, they lose their turn. However, I found that rewarding them for taking swift actions is far more effective. I give my players a +1 circumstance bonus per five levels to any rolls they make during their turn if they begin taking actions within fifteen seconds. I call this bonus "seizing opportunities." It works phenomenally. That, plus some recommendation for the players to plan actions during someone else's turn, means that they are likely to have their plans ready and in motion within those fifteen seconds. The bonus is big enough to be worth striving for, without being so big that it's not worth sacrificing for the occasional tactical overview. Players are much more likely to confer with each other this way. This has sped up our gameplay immensely.
You can adjust the bonus, time limit, and everything else to whatever works for you. I just insist that rewarding players is a more effective motivator than punishing them. It also makes them feel good and have more fun. That is the most important thing.
EDIT: Now that I think of it, this is also a pretty effective way to speed up offline tabletop combat, for anyone else who might be doing one or the other or both.
EDIT EDIT: One other thing I should mention is that when playing through an online tabletop you usually have computing tools available. Take advantage of them! The human brain is great at lots of amazing stuff, like creativity, imagination, reason, and emotion. One thing it has a hard time with is mathematics. Fortunately, computers rock that stuff. It helps to shave more time off of a turn if you have as much action content (rolls, initiative order, etc.) hard-coded beforehand. Dice macros are a simple example.
On a non-computing basis, it also helps to have actions listed out in front of the player. I know there was a lot of hate around 4e's action system, but having a sheet of actions clearly delineated in front of the player was a huge advantage for newbies. Having some clear options in front of them helps remind them of what they can do, and thereby lets them make a decision sooner.
Naturally, you do not have to be as hard-and-fast as 4e. You can even go as simply as reminding the player of what they can do in your game, or give them some soft suggestions.
A "campaign" isn't what it used to be...
Early campaigns often had multiple groups running within the same campaign. That is, the group of {Alice, Bob, Charlene, Dave, Edith, Francis, Ginny, Hal, Iris, and Jake} and the group of {Adam, Betty, Chip, Delilah, Edwin, Frances, Garth, Harriet, Isaac, and Jessica} and the group {Alice, Adam, Karen, and Luke} could play in different sessions but in the same world: all in one campaign.
From the AD&D Dungeon Master's Guide, italic emphasis mine:
Time in the Campaign
Game time is of utmost importance. Failure to keep careful track of
time expenditure by player characters will result in many anomalies in
the game....
One of the things stressed in the original game of D&D was the
importance of recording game time with respect to each and every
player character in a campaign. In AD&D it is emphasized even more:
YOU CAN NOT HAVE A MEANINGFUL CAMPAIGN IF STRICT TIME RECORDS ARE NOT
KEPT.
-Gary Gygax
You didn't need to keep track of time for your group, you needed it for each character. The actions of one group might redound to the benefit or detriment of another group. You might have them racing to achieve the same objective... or working at cross-purposes.
Consider, for example, these players' anecdotes:
I played in a couple campaigns in the 1978-1982 time frame that were run this way -- one DM and 3-4 groups of 3-6 players who met on different evenings and never encountered one another. Required a pretty dedicated DM.
-user Zeiss Ikon, comment of 3 Jan 2017
I once ran a "Fall of Myth Drannor" campaign, it had 15 players spread over 4 groups. Some players played different PCs in separate groups (around 24 ~26 PC total). As the campaign ramped up and the mortality rate did too, the groups were merged, we ended up with a single group of six people. I have to agree that timekeeping is paramount when dealing with such campaigns.
-user Mindwin, comment of 4 Jan 2017
Run this way, it's not too hard to imagine scores of players involved in the same campaign.
Best Answer
Ask your players how they feel about it
It's possible they don't feel any slowing down (or are ok with it). Maybe a big group isn't a problem for them.
If they want to stay as one group, ask them to help you to make the game flow better. That include:
You can also prepare yourself before the session. For instance:
Have the battleground and the 'flow of battle' ready before the session
Have all informations quickly accessible. I personally have a paper with a short abstract of every NPC to be in the battle. All the attack and defences values, HP, equipment, special abilities... in the same place, always visible to me.
Since you're on Roll20:
If they also feel they're too many, you can split the party.
You can for instance play the two group in the same campaign, with one group going after one half of a McGuffin, while the other track a group of bad guys wanting to use the other half of the McGuffin to gain power beyond imagination.
An other possibility is to split the party, and run the same campaign twice. It greatly reduce your preparation load (even if they follow very different path, you can reuse assets like map and NPC without the players noticing), but you get other problems like not remembering which team did what in that village, or spoilers from one team to another. Careful note taking and a simple explanation to your players should avoid both those issues.
In both case, you need to see about your schedule. Do you run twice as many games? Will it be one group one week and the other the next one? It's for you to decide as the group.