They could have trivially destroyed Earth ships, if they were fully active and adapted, and if that was what Sheridan wanted.
The Vorlon skin of the ship adapts to weapons as it "learns" about them - this was shown most especially with the Shadow vessels, which lost effectiveness against the White Stars as the war went on. Sheridan was surprised at the ineffectiveness of one of their weapons at one point, and had to ask about it - I believe it was Lennier who explained, in that episode. (Unfortunately, I can't find which episode. Lennier's explanation ended with something like, "leaving just the kinetic energy to deal with", after the weapon energy was deflected)
However, Shadow weapons are different than Earth weapons, so the skin of the ships had not yet adapted, making them a little more vulnerable.
The White Stars are also partially based on Minbari technology, whose defensive capability seems to be primarily based on preventing opponents from locking on - which can be disabled as it was in Points of Departure, when the Trigati moved on the station. But Sheridan couldn't utilize this technology, thanks to the message he had sent to all Earth ships: They will only be considered hostile if their weapons lock on to a White Star.
They had to leave the Minbari jamming technology disabled so they could distinguish who wanted to defect from President Clark and who was with him.
A couple similar questions were posed to JMS (the creator of the series) when that episode aired, and he had this to say:
"It seems to me that the White Stars (particularly in the numbers I perceived in this Ep) should have been able to deal with the Earth ships like lamb to a slaughter."
Yeah, if all you want is to slaughter fellow humans and officers; if you want to try to minimize the body count of your fellow officers, who you want to eventually side with you, it's more difficult, you have to keep the kid gloves on...which endangers you in the long run.
"were these White Stars tired or something? I would've expected that a couple of them could just go slice-slice, sever the rotating sections on the EA ships and have 'em for breakfast."
Sure, if all you're interested in is slaughtering the opposition...they were designed for "destroy"...when you have to pull your punches to avoid just going in and killing EVERYthing in sight...which will only work against you in the long run, it becomes more difficult.