The InterstelerDefenseModule in StarbaseModuleDefs.xml found at C:\Program Files (x86)\Steam\steamapps\common\Galactic Civilizations III\data\Game\ has the MoveCost EffectType Values reversed compared to TractorBeamModule (ie -0.25 for enemies and +0.25 for you).
As a result, the Interstellar Defense Module makes your own ships crawl to a standstill while your enemies can zip right through.
No wonder the Arceans were conquered . . .
Editing the xml file does not appear to affect loaded saves, at the least.