These are PHP Notices. Errors of type E_NOTICE are usually not very serious. You should write your code to avoid them but a lot of widely popular PHP code is full of E_NOTICE situations. There are a variety of error types:
http://www.php.net/manual/en/errorfunc.constants.php
E_WARNING and E_ERROR are more serious.
If you want those errors to stop, you should be more careful in writing your code and the correct approach would be to rewrite your code to avoid E_NOTICE errors. If you want a shortcut, you can reduce the error_reporting setting in your PHP.ini file or use the [man]error_reporting[/man] function to alter the error reporting level in your scripts.
All that said, I don't think this would necessarily make your website slow. Slowness can be caused by many things: lack of RAM or overloaded CPU. Slow database queries, lack of bandwidth, etc.
