It's a trade off. Using libraries can save considerable development time, reduce maintainence costs and perhaps improve the robustness of your code.
Many PEAR classes bring in an alarming number of include files. But caching often makes this irrelevant. Ask yourself: Does the site run fast enough? If not then pinpoint exactly where the bottle necks are and focus your efforts on them. Time spent logging in is usually trivial.
And if you really do have a proverbial "thousands of hits every second" site then consider writing a php extension.
