You may reduce the speed of a Task Group if that is a concern; of course there's also the thermal reduction tech line.
How this works may not be completely intuitive though:
A 6000km/s beam TG moving at 3000km/s because it's following a TG of that speed will have its full TH output and use 6000km/s for combat calculations, but halve its fuel burning rate (so total range is the same).
Reducing its TG speed to 3000km/s manually doesn't affect fuel consumption, reduces its thermal footprint, but also means it'll be easier to hit and may have a harder time to hit fast targets.