Before you use the techniques below, you need to explain to the players that you would like to speed things up a bit and that it is OK if they don't always get the best possible strategy in a situation.
If the players hate the idea then think again, you may have the sort of group that likes ponderous play. Perhaps suggest playing this way for a few sessions because they might like this too.
Tell the players that they have X minutes to collectively make up their mind
Perhaps not appropriate for your group, but helpful to others!
When the time runs out then stuff will start happening around them as if they are not there. Start a stop watch (say on a cellphone) and put it where everyone can see it. If they are still fussing, start adding modifiers, making it more difficult for them to achieve it.
Have dice and stats at hand before combat
Looking through books and character folios takes time. Have them open and ready. Photocopy out common combat spells so there is little reference.
Let them be late to the party
Use this one sparingly. If the players take a long time planning (after you've warned them), when they show up to the event, have it in a state that it's already progressed. If goblins are raiding a village, then the village should be on fire when they arrive, most of the goblins having already cleared off. There is still a fight, still people to save but it's not the joyous win they might have hoped.
Reward them if they are early
Opposed to the one above, you can let them steal a march on the enemy by not fussing and being early. I would make the rewards narrative, to make their victory sweeter or give them new allies.
Drive with a friendly NPC
Inserting an NPC on the player's side can give them a bit of motivation. The NPC can then be used to "do things first" to help scrub possibilities off the many connotations of plans. I also use NPCs to do the silly stuff the more risk-averse players are afraid to.
Drive with a neutral NPC
When dealing with NPCs that are not on the side of the players, it can be useful for them to be impatient. The village blacksmith "hasn't got all day!".
Give an in-game time limit
If untold evil comes out after dark, the party might feel pressed to get into shelter when the sky turns orange. You can do this by giving clues while they're planning. "The sun is behind the clock tower", "The clouds have turned orange" etc. Subtle hints are the way to go, it's up to them to make the logical connection.
You (as a GM) must be ready
Have all your events, encounters and stuff ready beforehand. If you slow the game down, then there's nothing the players can do. The more organised you are, the easier it is to keep the players busy.
As the GM, if you get excited and talk faster then they will too. You might stumble over words, that's OK, the action demands roughness! If I know a certain section is coming before the session, I will write it down in notes. That way, when I come to it, the words are already in my head.
Have the next plot hook dangling
If the players know what to do next, they're much better at getting on with it. Even in a sandbox game, you can offer a broad number of obvious choices for the players to do. If they don't feel like they have group (or personal) objectives then they will tend to lag. This can be difficult in a dungeon crawl where the objective is loot. The loot's not going anywhere!
Make the plots interesting
If the plots are interesting, the players will be desperate to find out how it ends. They will force the game along to find out what happens next.
Drive plot between sessions
If you can, the downtime between sessions is really useful to wet the appetite of the players. When they get to the session, they will want to find out what's next. Use the downtime to dangle little tidbits of information that they can't discover without being in-game.
Help them make choices
Where appropriate, you can give them advice. Particularly when buying stuff as that can take ages. Your knowledge of the game books can help suggest the place to find the right sword or the most appropriate armour.