Session Variable performance? vs. Cookies

First off, sorry to be vague on this, but I'm going on word of mouth on this one. "I heard someone, somewhere, talking about..." sessions impacting the performance of their web server. They had setup a website (I think they were using MS & ASP) that used session variables to track visiting users. Once their usage reached about 500 users, the web server slowed to a crawl. They rewrote everything to use cookies, instead of session variables, and now they're running fine. They claimed that the web server was having problems managing all the session variables (for all the current users). And they suggested that no-one should ever use session variables for large applications. Is there anywhere I can find out more info on this? Could is only be a MS/ASP thing? (I wouldn't be surprised:) Has PHP been benchmarked with lots of (500, 1000+) user sessions? I'm using PHP4 & Apache. From what I've noticed, PHP stores the session variables in text files (in the /tmp directory). I could see some slowness, if PHP actually has to call the file every time the variables are accessed. Or if there were, say 1000, session files in the /tmp directory. I'm just wondering if anyone had some insight on this. Apologies again for this sounding vague. Thanks, mario