At lv 1, Yorick ghouls have a speed of 300. Simple and understandable in some cases of 'Yorick should be the one who can get closer to you while ghouls are for point blank summoning damage.'
By lv 9 they can run as fast as champions with 345 MS with tier 2 boots. If the Pestilence ghoul is chasing you then anyone with 355 MS will probably get slowed enough to get hit as well.
By lv 12 they have 443 MS. Unless you are Master Yi you are not going to outrun them.
You can hit them but remember, they are small, clump together, have hard to click on hit boxes and also Yorick will be right on your tail as well with his Omen of War speed boost.
It's quite silly for someone who was disabled because 'he was too much of an HP sack,' to have pets that can run faster then most champions in the game, deal quite a good chunk of damage and actually stick to targets better then Tibbers, Shaco's clone and voidlings.
The ghouls can have some late game speed to catch shiv/ludens users but just not speed that can catch an Alistar using mobility boots.
(That would be 435 MS)