The Shapeoki wiki page is a good starting point, but each machine is different. I would suggest you do a feed/speed test similar to the one at Calibrating Feeds and Speeds When Using Carbide Microtools. You will need to do it for each different bit, in each different material it will be used in, because two similar bits may be drastically different in the same material. For instance, on my machine the 1/8" straight flute flat end bit is best for fast, shallow cuts, while the 1/8" upcut ball end bit is best for slow, deep cuts.
The 611 was not designed for hours of continuous use, and having it set so high will wear out the brushes very fast. Most wood can easily be cut with a setting of ‘1’ or ‘2’, which will increase the brush lifetime and prevent overheating of your bit.
Finally, I’d recommend making sure your belt tension and pot settings are good. I posted a couple videos in this forum to help with them: X-Carve Maintenance/Troubleshooting Videos - Add Your Own!