Basically here are a few slap bass riffs that should improve 
your thumb speed.
Since I'm the only one who will really know the true timing I have 
made them have similar timing to other riffs/sons.

This one is very similar to higher ground.

G————————————————————————————————————————
D—————7——————————5——————7———————9—————8——
A————————————————————————————————————————
E—5—5——————x—x—x————5—5———x—x—x———x—x————

This one is similar to the FF7 boss tune (if you don't know it it 
doesn't matter you can make up your own timing)

G————————————————————————————————————
D———————2————————————————————————————
A———————————————3———————2—————1———0——
E—x—x—x———x—x—x———x—x—x———x—x———x————

This riff isn't mine but it's still a good one to know. It's a riff 
from Primus' version of Master Of Puppets.

G—————————————————————————————
D—————————————————————————————
A—————2————3————4————3———2h3——
E—x—x——x—x——x—x——x—x———x——————

And also the slap riff in Get Up and Jump by RHCP. (I've tabbed 
the entire song out already) So look at that if you get the chance.
Tom