30s is significatly shorter but 6 min is not, we are talking about the same order magnitude (60%) while 30s is obviously significally shorter(5%), You are just being nitpicking I'm sure if it was 8min instead of 6 you would have said the same thing, also I can return the same question to you, how many new scrypt altcoin you can count that have a 10 min block time? You have your answer...
Actually, the % limit doesn't help much at all with convergence, even if you assume a fixed hashrate, because the difficulty spends so little time in the narrow band where there is usable feedback: Here's a graph projecting difficulty with fixed network hashrate of 300 MH/s:
Simplified explaination : lets take the example of when the diff is increasing, the 12% limite makes move in 12% steps per block now lets imagine we have said hashrate that it is supposed to push us to diff = 100, the diff will start increasing in 12% steps trying to reach that diff, but at some point when the diff reach a certain level lets say diff=60 the coin is no more profitable and the profitabilty pools leave so the peak we've got is diff 60 instead of 100 thanks to the fact that diff increased in limited steps and the diff didn't reach the top value it was supposed to reach thanks to diff retarget each block (since with the next block the diff target will be significally lower and the diff will start decreasing). Same thing will happen when we are going downwards when the coin will reach a certain point where is profitable again (in 12% steps) lets say diff 20 (while if there was no limite the diff should have gone down to 1 for example due to low hashrate) so each time you'll have the coin bouncing with smaller minimum and maximimum and converging towards the diff (limite) on which the coin is profitable (No this doesn't take into consideration the 36 average and this is what's make everything jerky in addation to everything I mentioned before). So maybe removing the 36 block avergae can solve the issue if the other parameters don't swing in a major way
I've included this in my model. As difficulty goes up, I reclaculate hashrate dropping off exponentially, which is a reasonable assumption if you look at the pool hashrates here: http://cat.coinium.org/index.php?page=statistics&action=graphs
In my graph on the previous page I assumed that 1GH/s worth of profit pools would jump in at any difficulty below 45, which is about where the current price/profitability point is. That's a bit arbitrary, but as the graph shows after 4 cycles the difficulty just stays very slightly above profitability... which is exactly what we want. It would do the same if we assumed profitability occurred at 30 or 50 or 100.
Sadely this is a wrong assumption and I understand the reason behind this, but it doesn't work like that, Consider the huge part of the increased hashrate from profitability pools as an On/Off Value because these pools switch instantly all their hashrate from a coin to another (switching ports dynamically for everyone at the same time) there is a minority that switchs coins manually while it's random it can be assumulated or interpolated as an exponentiel function