I fly the B737 but I imagine the speedbrakes on the A320 have approximately the same performance. i.e. they reduce the lift/drag ratio by about 30% when fully extended.
Some observations that I have made over the years that have served me well...
Whatever your descent speed is whether it be 210 kt, 250 kt, 280 kt, 300 kt, or 320 kt, when you extend the speed brakes, the descent angle will increase by 1 degree.
e.g. At 210 kt IAS your descent gradient will increase from say 2.8 degrees to 3.8 degrees.
At 250 kt IAS your descent gradient will increase from 2.9 deg to 3.9 deg.
At 280 kt IAS your descent gradient will increase from 3.0 deg to 4.0 deg.
At 300 kt IAS your descent gradient will increase from 3.2 deg to 4.2 deg.
At 320 kt IAS your descent gradient will increase from 3.6 deg to 4.6 deg.
If you use the 1 in 60 rule, 1 degree = 1 nm per 60 nm
1 nm = 6000 ft (actually it's 6080 ft)
therefore = 6000 ft per 60 nm = 1000 ft per 10 nm.
In other words, at virtually all speeds, full speed brakes will increase your descent profile by 1000 ft for every 10 nm.
So if you are 2500 ft high on profile, if you remain at the speed programmed in the FMC, it will take you 25 nm to get back on profile. (Give or take for the effects of wind. More distance with tailwind, less distance with a headwind).
Now you could keep thrust at idle, increase airspeed, and the aircraft will zoom down to a lower profile, but the total energy will remain the same. You will simply be converting potential energy into kinetic energy. Your descent angle will increase a little once the airspeed has stabilised at the higher value, from say 3.0 deg to 3.6 deg but you will have to slow down again at some stage and this will convert your increased kinetic energy back into potential energy.
You will hear people say that speedbrakes are not effective at 210 kt but this is not true. It's just that because you are flying at such a relatively slow speed, the effects seem slow too. But I bet you if you were on final approach at 20 nm and you were 1000 feet high, speed brakes alone would have you back on glide slope at 10 nm to touchdown. (i.e. 1000 ft per 10 nm) It's just that it would take around 3 minutes to do so which would seem to take for ever! But don't forget you're only travelling at around 3.5 - 4 nm per minute!
P.S. The descent gradients given above are not absolutely correct in that they depend on the actual gross weight at the time. As weight increases, the best L/D airspeed increases, therefore those gradients for a given airspeed would vary a little from flight to flight as the gross weight varies from flight to flight. But the extra 1 degree descent gradient is pretty consistent give or take 0.1 degree.