This was calculated by Xcano on Naruto Forums before (here). That was the old calculation method though. So I'll try to recalc it. Still, credit to Xcano. Without his calc, I am unable to calc this.
So according to the Explosion Speed Calculations page, we have to determine the type used first. It doesn't seem like C4, so I'll assume TNT.
Second, we determine the weight of explosive used. The bomb's not visible, so I'll use Method 3.
D is diametre in feet. W is weight in pounds.
So according to Xcano's calculation (link at the top of this blog), the fireball radius is 96.52257 cm. So we multiply that by two, we get that the diametre is 193.04514. Converting that into feet, it's 6.37048962 feet. Plugging that into the formula we get that the weight is around 0.43 pounds, or roughly 0.2 kilograms. Using this method, the explosive assumed should be TNT.
Then we'll just plug everything into this calculator. The explosive type would be TNT, the weight of explosive is 0.2 kilograms and the range (radius) is 0.9652257 metres, as calculated by Xcano. The time of arrival we get is 0.69, in milliseconds. Supersonic+ reactions.
So that's 0.00069 seconds. As Xcano had calculated, Scarlet Witch moved 112.04545 cm to block the explosion. That's 1.1204545.
1.1204545/0.00069 = around 1623.85 m/s, or roughly Mach 4.77.
Supersonic+ MCU here we come. RIP Forever Subsonic MCU.
Supersonic+ combat speed & reactions.
As said in the comments, it is unknown whether if Scarlet Witch reacted to Crossbones pressing the button. If that's the case, that's still Supersonic via calc in the comment.