Stevenage looked lacklustre after an enforced three-week lay-off while Orient looked anything but with Cox, along with striker Charlie MacDonald and new arrival Shaun Batt - who joined on loan from Millwalll this week - all impressive.
Orient sit just five points outside the play-offs and Slade believes his side have come a long way this season.
"We defended very well, they're a big side so they're a problem from a corner or a free-kick but we managed to keep them out on that front, and at the other end Jamie Jones wasn't troubled too much," he said.
"I wasn't surprised at Shaun Batt's impact. He's got pace and he's an awkward customer, he was a constant threat throughout the game.
"He showed a very good understanding with Charlie MacDonald. That shows what a good pro he is - he wanted it to work.
"Charlie's worked tirelessly, he's done everything but score, his work-rate is phenomenal.
"We deserved to take the lead through Dean Cox, he can get a goal, that got us on the front foot and that was great.
"The substitutes we were able to bring on shows how far we've come, we're turning into a very solid League One side. If we can just push on a little bit more and add to what we've got we're only going to go forward."