The difference between what is a 'speed' board and what is a 'small slalom' board overlaps a bit depending on the size of the sailor, but the essential characteristics are a bit different.
To me a 'speed' board is one that is designed for maximum speed on a flattish and broad course in strong winds of 28-30 knots +. It is not designed to float you and it has no compromises to make it better to Gybe. It is not designed to be fast on a beam reach (although in flat water it can be), but is optimised for speed on a broad run.
All this means it is narrower overall, had a proportionately narrower tail and lower nose rocker than a slalom board.
A pure speed board is not going to be faster for Alphas than a slalom board except in the strongest winds on flat water where a slalom board will be getting flighty over 30 knots beam reaching and hard to sink into a turn at well over 30 knots of speed. It will be much harder to keep speed out of the gybe and accelerate but a highly skilled rider in very strong winds may make it do this OK.
For me at 75KG and 176cm a speed board is under 60 litres and under 44 cm width. (CA40). Anything 45cm to 50cm is too large and slower to be a 'speed' board for me and not as good for "slalom' type use.The difference on the speed course is like getting another gear and amounts to 5-10 knots of top speed. Having said that, I have had a lot of fun on 45-50cm wide boards in 25-30 knots winds, but the smaller boards are always faster for me by knots on the ideal course!
For me, a 'Slalom' board is somewhere around 50 - 60cm wide and 80-90 litres. My all time favourite is the IS87 for winds in the 16-28knots range (sails 5.4m to 6.6m). For 12-16 knots winds I need something about 100-110 litres and 68-70cm width (6.6m to 7.5m). The slalom board is faster around a slalom course, faster through and out of gybes, can almost float me for an emergency uphaul, has given me most of my best Alphas up to 27 knots and will go high 30's on the speed course regularly. I have almost accidentally got over 40 knots peaks on it a couple of times in fluke conditions. If I am on a 5.4m sail and do a high 30's run on the slalom board, I will go over 40 knots immediately when I switch to the speed board. I can run the slalom board at 30 knots through fairly mild open water chop reasonably confidently whereas the speed board would be out of control, or very hairy! I rarely use a fin deeper than 20cm in the speed board or a sail larger than 5.4m.
The exception is that I have used my older, wider SB IS50 speed board (46.5cm wide x about 50 litres) for my best 1hr on Lake George (28.8kts) and one of my best at Sandy Point with a 22cm fin and 5m sail on 30 knots of wind where the larger slalom board was a bit too much for me and that wind and speed. I also did my potentially fastest Alpha 500's in that 1 hour in the 28kts+ but the radius/proximity was outside the 50m limit! In the ones where I made the proximity circle I lost more speed.

The extra width and slightly wider tail on that board made it easier to run a slightly larger fin and keep speed through gybes than I could on my CA40. Top speeds on that board in the pure speed course downwind at Sandy Point were 2-4 knots lower than the CA40 but may be closer with the latest fins.
I must emphasise that the size of what is a 'speed' board and what is a 'slalom' board is directly related to the size of the rider and there will be little or no advantage if it can't be sailed broad, in strong wind on flattish water. But is is more than just the size or width of the board that separates the two types.