Of course a block time shorter than 10 minutes is perfectly viable! I never said that it wasn't. My point was that there exists a point where the orphan rate caused by a shorter block time ends up outweighing the benefits of a shorter block time. This starts to matter earlier than you may think because an orphan rate of just a few percent would be enough to encourage miners to centralize. However, ignoring that, here's an extrapolation of the reduction in effective hash rate as a network the size of Bitcoin reduces block time (the Bitcoin network currently sees about a 1% orphan rate):
| Blocktime in seconds | Reduction in effective hashrate |
| 600 | 1% |
| 300 | 2% |
| 150 | 4% |
| 75 | 8% |
| 37.5 | 16% |
| 18.75 | 32% |
| 9.375 | 64% |
As you can see, two and a half minutes per block isn't actually all that bad in the reduction of effective hashrate, but I wouldn't want to go much lower than that.
How does your table prove this? Where is the formula? What's this effective hash rate nonsense. We have block and timestamps on those block we can check to see if we are getting them at roughly one ever 15 sec.
If a 15sec block rate didn't work because of orphans my WDC mining stats would tell me this and they don't, in fact WDC is working really well, and is doing transactions faster than any other coin I have tried.