The speed in "normal" Transwarp is very limited. But after the assimilation of an specie - who had another drive system called "quantum slipstream" - both drive systems were integrated by the Borg and the new Transwarp were incredible faster.

The speed of "normal" Transwarp is very limited. But after the assimilation of an species, who had another drive system called "quantum slipstream", both drive systems were integrated by the Borg and the new and improved Transwarp was incredibly faster.