Really, the slower a unit is, the more it needs to depend on its enemies being nice and coming to engage it in order to be effective (because it certainly won't get far trying to chase them in turn), and the easier it will be to hit by comparison while finding them potentially harder targets. 4/6 vs. 3/5 may not look like much, especially on small maps, but the 4/6 -- meaning the Hellstar in this case -- can basically "waste" one MP per turn on such things as facing or elevation changes or more-difficult-than-clear terrain before it no longer breaks even on the AMM vs. own TMM comparison where the 3/5 has no such reserve, and it can potentially reach cover or other favorable terrain that little bit more easily as well.
Also doesn't slow down any buddies it may have brought quite as much, of course.
Now if the Hellstar absolutely has to engage your 3/5 siege engine in sufficiently cluttered terrain that it can't easily play the range game, then it's potentially in trouble because its mobility advantage isn't quite pronounced enough to reliably get into your rear arc (only takes 3 MP to do an about-face, after all) and your up-close forward firepower may well exceed its own. On the other hand, if your objective is to take it down and the Hellstar is in principle just as happy to evade your big guns all day long and instead simply take potshots at whatever softer targets of opportunity present themselves...that's another fight altogether.