Cntry, there may be too many variables for a streight forward answer to your question. Your body weight, how tight you keep the knot, how much friction your tie in point generates, and what rope you are using for instance. But I climbed on a Blakes on Arborplex for a while, and my answer to your question would be "Fast enough for work within the canopy, but not fast enough for descent to the ground". Particularly not when bleeding or pursued by a nest of angry wasps. Of course, in those cases, nothing is fast enough, but the Blakes is way too slow.
I do a good bit of pruning in the tops of mature loblollies. When I get ready to come down I dont like to mess around. I keep a figure 8 on my saddle and let it take the heat of the rappell. Its faster and more fun.
You can usually feel and smell the heat building up in the blakes quickly enough to stay out of trouble, but I recently had a freind tell me he loves the smell of burning poly. Not me. When I was climbing on the blakes I learned pretty quickly to (1) check it frequently for glazing and broken strands and (2) use a split tail system, so I wasn't constantly shortening my climbing rope with cutoffs.
One other caution from personal experience. If you use a splitail of a different kind of rope than your climbing rope be sure it is suitable for the pourpose. I burned through a piece of kernmantle once that luckily, but barely, got me to the ground before it came apart in my hands.