[PaperclipTaken]

"A common misconception is that transactional confidence is based on time"

Transactional confidence is based on the probability of someone compromising the network for enough blocks to undo a spend that you've already honored (IE you gave them what they paid for, then they undo the transaction). At 6 blocks, you are assuming that an attacker won't have the computational power to reverse 6 blocks in a row.

When blocks come out every 2 hours, that means you need to find every single block for 12 hours, or essentially control the blockchain for a long time.

When the block rate is down at 5 minutes, you only need to control the network for 30 minutes - you have more chances to get lucky and undo 6 transactions in a row in your 12 hours of purchased computing time.

A halved block rate does not mean you need less computational power, but it does mean that computational power is probably cheaper and 6 blocks worth of power is going to be more affordable.

[makomk]

Unless your purchased compute time is more than 50% of the network your odds depend mostly on the number of confirmations and not the time they take. The cost of a sub-50% attack increases exponentially with more confirmations but only linearly with longer confirmations, so the number of confirmations is more important.

[muyuu]

5 minutes is still a perfectly acceptable block time. It only gets problematic as network latency can be significant in block discovery chances. This happens under a few seconds per block.

There are alt-coins working with 1 minute blocks and they behave just fine. Provided hash power is spread among the network, there is absolutely no problem with this, and it won't be long until we go back to 10 minute-ish blocks.

[MacsHeadroom]

A successful attack requires more than simply reversing any 6 consecutive blocks. It requires reversing a specific set of blocks containing a transaction one wishes to be reversed.

This is why 5 minute blocks are virtually every bit as secure as 10 minute blocks.