the question at hand does not involve speed in anyway whatsoever,so stop riding and start typing
what you are saying is he summoned a creature as big as a bijuu after evading the c4 karura and Deidara couldn't spot it from above at all!?
It does, because it involves chakra, speed and dexterity with Jutsu. Sasuke already outran a bomb's shockwave without a scratch, covering a distance much larger than Minato did in his best manga speed feat:
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If you knew how fast a detonation shockwave travels, then you would understand how insanely impressive Sasuke's speed is. In fact, not until EMS Sasuke tiers show up, this speed is unbeatable bar Itachi's. He was never out of chakra. That was something Deidara assumed.
So, let's add all of this: Sasuke evaded an explosion by a sizable margin and evaded the second one as well. That alone is foreshadowing that he can. Genjutsu is instant and summoning itself is also instant as it's an S/T jutsu. How did he do it? He outran the explosion by a certain margin, performing the summoning along the way and then Genjutsu'd Manda and reverse summoned it.
Since Sasuke already showed he was capable of outrunning shockwaves, it isn't farfetched to assume he did that here as well, as the author already foreshadowed it that he can.
