You'll have to extract the Spaceunitscapital.xml, Spaceunitsfighters.xml, Spaceunitsfregates.xml and Spaceunitscorvettes.xml. Then open each with Wordpad or Context. Hit Ctrl+F and type Max_Speed. It should scroll you down to it. Edit it how you wish. If you want to modify the Hyperspace speed, Hit Ctrl+F and type Hyperspace_Speed, edit it how you wish. Don't forget to do this for all (capital, fighters, corvettes and fregates).