Here's another solution for slower computers:
Parent it "manually".
Not exactly easy.
But when I tried some of the above solutions,
my computer was suddenly way too slow to pose the armature.
Parenting it manually fixes this issue tho.
- In edit mode, manually select some vertices nearby a bone.
(for the object you want to parent to an armature)
- Don't parent anything yet.
- add these vertices to a vertex group.
- name this vertex group exactly the same as the bone they're near.
- do this for other vertices nearby other bones.
- now parent the object to armature with envelope weights. Not automatic weights.
- Now it's somewhat rigged. It may require more tweaking.
Yeah, it's not perfect.
But this method let's me pose the object without major lag.
I only had to do this for hair, so it wasn't too much work.
Edit: Before I manually parented it, I applied a decimate modifier using the planar option (8 degrees). This allowed me to reduce vertices without reducing quality, whereas the un-subdivide method immediately made it look too low-poly.