The League One leaders took themselves to within two wins of the title with their 1-0 win over Gary Naismith's team at New Bayview.
But it was a turgid showing from the Light Blues as they laboured through the motions for almost the entire match.
They only secured victory after Pat Clark hauled down David Templeton in the box with just seconds remaining, with captain Lee McCulloch punishing the mistake with his last-gasp penalty.
McCoist was unhappy with most of his players but insists he would take the same showing in next week's William Hill Scottish Cup quarter-final against Rovers if it guaranteed victory.
He said: "I'm a little disappointed and the players are too.
"But results are the most important thing and we kept going to get it. It's another goal in the last minute and the spirit amongst the boys is great.
"But apart from Templeton we lacked a bit of spark. David was the man who was going to do something for us - the one bright light for us.
"You want to play well, you want to pass the ball better, of course you do. But any team who win a title, and I don't care who they are, will go through performances where they feel they can do better.
"The fact the emphasis is being put on our performances is because we have a 21-point lead. If it was neck and neck and we were in another league, with the greatest of respects, nobody would be giving a monkeys.
"That's just where we are at the moment. I understand that. But hopefully we can now kick on and wrap the league up and it would be good to do it in a bit of style.
"But then, I may be contradicting myself here but if you were to offer me the same result next week I'd take it. You only get one bite at cup matches so there isn't the same emphasis on performances."
Templeton set up Dean Shiels for an early chance but the Northern Irishman poked wide, while Nicky Law smacked a post after half an hour.
But that was about as much as the Glasgow giants created until their late spot-kick.
The winner sealed their 25th league victory of the season and McCoist said he never doubted his skipper.
"I fancied our captain to score," he said. "In a situation like that you would take big Lee every time.
"There was no complaints about the penalty and it was just a case of Lee keeping his nerve. Thankfully he did."
Naismith could not fault his players despite their late collapse.
"It was a very sore way to lose it,"he said. "I knew we were into the last minute so to lose a goal at that time after how the boys performed is hard to take.
"I'm gutted for them because the effort and commitment they put in was tremendous. We limited Rangers for most of the game.
"But I have no complaints about the penalty decision. It was a clear foul. It's the last minute but a reckless challenge. I think Pat just panicked a little."