How can I lower latency?
Latency is a measure of how long it takes for a system to react. In Pd, we are primarily concerned with the time it takes for audio, MIDI, or other input to cause audio output. This is "audio latency". For reference, the audio latency between your speakers and your ears is approximately 1 ms per foot.
Under Windows, use the ASIO driver. You're out of luck otherwise.
Under Linux, with a recent version of the stable kernel (2.4.x), special low latency patches, some hard drive tuning, and ALSA drivers, average audio latencies is 1.5 ms. If you really load your system, a maximum audio latency of 3 ms can be reached. Pathological bad cases have been known to cause 8 ms latencies this seems like the upper bound. More information on low latency under Linux can be found at Benno's site on the subject.