Is there any way to slow them down through Spacemaster or anything?
No. You can give yourself faster ships with it though, or missile tech, or something else.
If they're missile ships (and judging by scarce information provided, that's missile ships) they will attack anyway. Your best bet is to try to make them unload their missile stocks on something useless (AMMs, fighters, commercial ships, colonies, whatever) and then strike.
If they're beam ships, well, get yourself some PDCs and hope for the best. Without missiles and range superiority, you can say goodbye to your whole fleet.