According to Byakuya, he just condensed the power of Bankai. There's really no description of what that means, but enhanced speed just seemed to be an incidental effect.
In relation to fights, I don't think comparatively fast Espada are problematic because saying "Bankai does X" isn't the same as saying "their Bankai is best at X." Anyways, I don't think his speed has been a primary issue against his opponents. Against Grimm, the primary issue was power. Against the Hiero guy, again, power was the issue. Against Ulquiorra, we have moments where speed is an issue, but the greater issue is his ability to perceive Ulquiorra's speed and then match the power associated (Which he does when he turns horrow). Aizen is Aizen. The rest tended to feature his speed (Or I'm just too lazy to look them up)
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