If you use session on every page of a heavy traffic website, will this significantly reduce the performance of your server? Or not at all?
Say a website with 50.000 hits a day.
~2000 hits an hour.. shouldnt be too much of a drag, unless you're doing something silly like filling the session with files.
What do you mean with filling the session with files?
if session variables are stored in RAM then its going to slowly bring down the performance of server and if it is on shared web hosting then the web host would haver restricted by allocation of m/y per web app and that might shut down your website for sometime when there is a peak traffic.
$_SESSION['user pic'] = file_get_contents(somefile);