Put the summary of the bug you're having here:

What I expected to happen was normal lag-free operation of the server and client on my machine irrespective of other bugs.

What happened was frequent and often fairly regular lag of the client and server for up to 10 seconds or so, happening approximately every 5 minutes for the server, and more often for the client separately. It results in me being unable to do things for that amount of time followed by a spontaneous resuming of tasks. Went to minecraftforum.net for help in a chat room, they were unable to diagnose the issue specifically. Has been prevalent since 1.7.2 or earlier (I know at least 1.7.2 for sure).

Steps to Reproduce:
1. Run Minecraft
2. Load world (literally any world)/connect to my home-made bukkit gui server
3. Wait for it
4. Check log, find "can't keep up" error and notice in-game unresponsiveness for 3-10 seconds at a time.