May I ask how you know it is only 42 in/s? That’s a lot of speed lost. Have you checked for friction issues? Also 8.8 lbs is really light for a VEX robot (I think the electronics, etc. are 6 pounds alone). You should be able to go up to like 15 pounds just fine with your drive speed.
Honestly as for having it move less, just put traction wheels to stop being pushed around, more weigh will only harm you imo.
If you just want to make your robot heavier go for it, but I’m sure we’d be happy to help if you had specific goals like you really just want your robot stronger, harder to push, and faster.
I would recommend checking this out though: Designing Another Quality Drive