oop?

For awhile i didn't touch OOP PHP, classes etc... because people talked about how slow it was, but now i'm working on a site that has to do things the exact same on page to page, but i think that going with classes and funcitons might be better, because we have to carry a lot of data from function to function. Is the speed/performance that much of an issue? the site i'm doing is www.dinowax.com it gets quite a few hits a day (100k-200k). i know a lot of speed things that i've seen are like... 1.4 seconds of php compared to a class doing it in like 5... but all my current scripts, which use classes all go in under 1 second, and thats when the server is under strain. Any explanation would be appreciated. Steve