Orient are now three points off second-placed Brentford having played two games more.
Slade told Sky Sports News: "It's just so sad isn't it in the first half that they just didn't get behind the team? I find that so disappointing. Then we even got booed off at the end by some of them. It's a joke.
"Some people are criticising me and saying that we've given up. As a manager, I've not given up, this team has not given up. I'm frustrated for the players."
Orient return to League One action on Saturday at home to Bradford City.