One of the great things about Numenera is that combats are short and fun. No hour and a half battles that drag on and on. But sometimes, even a Numenera combat can outlast it's welcome. This can happen when the players consistently roll badly. Or the creature level is too high. This can happen when you have a mix of tiers in the party. What challenges a tier 3 character can frustrate a tier 1. And if the tier 3s keep rolling badly...
In cases like this, I steal the escalation die from 13th Age. 13th Age is a wonderful game, and if you haven't checked it out yet, you should.
How does the escalation die speed combat? Let me quote from the 13th Age SRD:
Escalation Die
The escalation die represents a bonus to attacks as the fight goes on.
At the start of the second round, the GM sets the escalation die at 1. Each PC gains a bonus to attack rolls equal to the current value on the escalation die. Each round, the escalation die advances by +1, to a maximum of +6.
Monsters and NPCs do not add the escalation die bonus to their attacks.
Put your biggest d6 out on the table and let everyone add its value to their attack. At 3 and 6 it effectively lowers the difficulty by 1 and 2, but don't count this as an asset.
Story-wise, as the battle continues, the clever PCs are learning how to fight these creatures. Or the shock of entering combat has faded and their successes in past combats gives them growing confidence. Or they are getting more desperate. Whatever fits the battle at hand.
So when do you do this? You could do it every combat, like 13th Age does. But you don' t have to. When you notice a combat is taking longer than you want, just put the d6 down, and set it to the correct number. For example, if you're about to start round 4, set the die to 3 and tell all the players they get to add 3 on all attack rolls, which drops the difficulty of the attack by 1.
If you do use this for all combats, balance the player bonuses by adding an extra point of armor to your creatures. This will result in fewer one round combats as well as fewer four round combats.
Next, see Escalating the Other Side of the Table and GMing Action Scenes: End with a Bang.