How fast is too fast for a 1/4" bullnose bit in HDU?

I'm working on a personal project and am cutting some 3D characters from 20lb HDU. For the roughing pass (1/2" end mill) I was running it at 6 IPS and am doing the same for the two finish passes (1/4" and 1/8" bullnose bits). All bits (other than the 1/2" end mill which came with the shopbot starter kit) are Amana brand. I had already run this project but neglected to allow anything under my 3D model and so it broke free and the job got ruined. My own fault I know. Was I just lucky with running at that speed that nothing broke? The bullnose bits are nearly brand new, this being just the 3rd or 4th thing I've cut with them, all being cut in 20lb HDU, 1.5" thick if that matters. Could I run it even faster for the finishing passes or am I already pushing the envelope?

The bits aren't the concern with HDU, it's your machine. As long as the speeds match the feed, and your getting a smooth cut, run the machine up to it's limitation.
When cutting 3D, your feeds aren't running at what they are set at. The feed is limited to the Z axis. Most people don't realize, the Z axis retards the feed, and doesn't reach full velocity.
Be sure to add a zero plane to prevent cutting through to the spoilboard and allowing the project to be cut free, or use tabs, or tape to keep it in place.

If you run to fast, you'll notice because the controller and the computer will loose sync and you'll get the ever dreaded "lost comms" dialog. When cutting HDU, I do tend to run my machine as fast as it will go, RELIABLY, without loosing comms. That is the main limitation with ShopBot machines. the communication speed between controller and computer.