3.1 is fine for just about everything. I only have one 3.9 machine, and it rarely gets booted up.
3.9 isn't that heavy though, the slowness is mainly related to running a huge jpg as your wb backdrop and selecting a screenmode with the maximum colours ECS can provide..
Dial it down to 8 colours and turn off the background pic and it runs quite nice. You can make 3.1 sluggish with the same choices. :-)