Byron Moore equalised for the vistors after Lee Cook fired Orient, who had already hit the woodwork twice, into the lead.
"For me, we should have been four or 5-1 up in the first period," said Slade.
"I thought that we were absolutely excellent, we were forced to make changes but after some of the football we played and some of the openings we created, the game should have been out of sight by half-time for sure.
"That's a lucky side in that away changing-room at half-time knowing that they had gone in at 1-1 and they are still in a football match because they should have been dead and buried.
"Our new player Charlie MacDonald was only denied by a post on his debut, I don't know how that stayed out and we've hit the woodwork twice. Their keeper was their best player."