6 minute block time is actually a wise design parameter choice by FreeTrade, and indicates dedication to having a strong network, while still being somewhat improved over Bitcoin's 10-minute time.
You cannot just randomly twiddle key design parameters and expect everything to keep working. (not that it's stopping anyone in most of the altcoins, of course
Neither can you just randomly compare different coin designs. Primecoin has PPCoin's *distributed checkpoint system* which offers a 2nd layer of security (along with its own plus/minuses), and is probably the reason SunnyKing felt 1 minute was long enough there. BitCoin does not have that, thus neither does MemoryCoin which derives from it. (it has checkpoints, but of a different kind.)
The strong advantage of a longer block-time is that it gives plenty of time for each block to percolate through the whole network and all nodes to settle and agree on a coherent distributed picture. This improves network stability and resistance against certain types of attacks. (For example, with short block times it becomes much easier for an attacker to get in front of the network, out-orphaning everybody because they cannot even "see" the front of the blockchain, let alone mine on it. Attacker does not even need a sustained 51% to do that, just "get lucky" a few times in a row.)
The reference below estimates potential end-to-end latencies in the "tens of seconds" = say 15-40 seconds, that's just for the network transmission. Apply a 10x factor for security, reliability, processing, databasing, actually getting in a little mining... and you're at 10x (15-40) = 150-400 seconds = 2.5 to 6.6 minutes.
So we've got LTC at 2.5 minutes, MemoryCoin at 6 -- both reasonable. I could see MemoryCoin maybe at 3 minutes, but it's not a clear-cut improvement, rather a trade-off.
The 30-60 second coins are really operating right at the margin of what is possible. And they have not been proven against attacks (on the contrary, several of them have been exploited). And they strongly advantage the clustered GPU farms, because while everybody else is spending much of the blocktime just trying to receive the blocks, the clusters have tight internodal transmission and thus proportionately more mining time as well. (this is why small/solo miners often get continuous orphans on these coins). So the longer block time in MemoryCoin does help solo miners. (Also remember, these are average, not exact blocktimes. At 3 minutes, some blocks will take only 1 minute; at 30 seconds, some blocks take only 5 or 10 seconds, and lots of miners never even got a chance.)
Another issue with short block times is they are just generating a lot more data = significantly increased overhead. Especially early in the life of the coin, the blocks are nowhere near full capacity in terms of the number of transactions they can hold, so why spew out dozens in place of 1. Remember a main point of these coins is to carry transactions.
See also:
https://bitcointalk.org/index.php?topic=211535.20the block time needs to be more than several times the maximum end-to-end propagation delay. This is needed to prevent competing nodes from generating blocks at the same time and convincing large parts of the network that their respective block is the winning block.
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So what is the maximum end-to-end propagation delay? Bitcoin networks are constructed by randomly connecting to IP addresses. This means that Bitcoin networks ignore geographic locations, so end-to-end communication can cross the globe multiple times. The network construction also does not limit the width of the network, so end-to-end communication can require numerous hops. Add processing delays on each node and you can easily get a maximum end-to-end propagation time that is tens of seconds.