Earlier this week, Orient rejected an official approach from the Bluebirds as they look to replace Ole Gunnar Solskjaer, and Slade, who has seen his own future become uncertain in recent weeks, has admitted that it will be a big opportunity for whoever takes over at the Cardiff City Stadium.
The 53-year-old is quoted by BBC Sport as saying: "It's frustrating. It's been turned down and that's all I can say, and whether Cardiff come back again, I'm not sure.
"Maybe they'll move on. It's a big club and it will be a big job for somebody that eventually ends up with it."
Slade saw his team lose 1-0 to Sheffield United in the League Cup third round on Tuesday evening.