Lighter weight does generally mean higher speed, however it's not a guarantee - if you're too light for the tip to get good traction, you'll go slower, and for attack types, if you're too light to handle the recoil you suffer even with the speed (movement speed also helps deal with recoil), then that speed isn't going to be worthwhile.
Even in plastics, there are cases where a combination can be too light to handle certain opponents even with speed, but generally as plastic is lower recoil, going for lighter weight for higher speed is generally good. However, it's worth noting that despite being the most externally distributed weight disk, few attack types can use Wide Survivor simply because it's too light, so even there you have a balance that you need to get right, it's just a lot more weighted towards speed over weight.
In MFB, recoil is much, much higher, so the balance tends to lean a lot more towards being heavier rather than faster (and it's also worth mentioning that weight distribution plays a huge part in speed as well, so heavier but wide, edge-focussed wheels *can* move faster than lighter ones. It is still all about finding the right balance though, and in Limited, LLD's speed is IMO a big part of its power (especially because half of its contact points are plastic).
Galaxy, however, will never find use. I actually tried it again yesterday trying to get anything out of it and it was completely ineffectual against even CS-tipped defense (which aren't even legitimate defense combos in Limited, instead CS is a balance tip) with a height advantage and everything (was aiming to make use of the contact points on top of the wheel). Completely awful part.