Leftover time is not added on to the next level- I wish it did.
I regularly make it past level 10. This is what I do:
Before the level even begins, start by positioning your cursor over the upper left hand side of the screen, then click cards as pairs. Repeat the first four over and over in your head like a mantra (ex. "gorilla, rabbit, elephant, green monkey"), and continue clicking through the cards. Don't wait for the cards to turn back around, go ahead and start clicking on the next pair.
Some games are easier than others- but just keep practicing.