That's nice to know but.. are we talking of walk speed, or build speed? I assume you mean build speed, which means a work mule is about 50% faster than a normal worker, right?
I find it rather easy to get them by the way, if you use your hunters aggressively enough you will usually manage to get some animal that you can use to make a relevant herd.
On the other hand my continent, which is huge and has 8 civs, has NO cow, pig, sheep. Kind of, you know, a bummer

Those resources are really hard to do without, and you can't hope to get them through discovery like you can with vegetables at farms or minerals and metals at mines.