They said the same thing about Majunior's giant form, which was that they became humongous but remained fast as ever. I really doubt Toriyama intended a speed boost yet wrote a statement just exclaiming they were still fast to prove it. It may provide a minor speed boost if we were to assume Goku wasn't factoring in Vegeta's ki loss from making the power ball, but I doubt it since Goku can sense ki and remarked his ki went down upon creating it.