This is a withdrawn posting.
I think it all depends on your target audience. a fast food establishment or convenience store might require speed over privacy but a financial institution that regularly transfers large sums might not even consider a transaction time requirement over surety.
Getting through confirmation transactions quickly I think is going to be the key to any currency's success when it gets to a certain market capitalisation.
I read this weekend that Bitcoin has over 200,000 unconfirmed transactions pretty much on an ongoing basis and the transaction fees are steadily increasing with people wanting to avoid the queue and pay a higher fee to be confirmed quicker.
So anything that can be done to make it faster would be ideal - for scenarios like @GPU_Mining mentioned.
Just creating the zk proof for a shielded transaction (which is the real reason for zcash to exist, if you want transparent transactions, there are better options) already takes a couple of minutes.
Also, considering a transaction to be settled after just one confirmation is probably too risky for any non-negligible amount (and with faster blocks, that risk would be higher, so you would want to wait for more confirmations).
So, summing up, a shielded transaction with 2.5 min between blocks would need about 4-5 minutes to get a very low level of security; maybe 15 minutes to get an acceptable level, and an hour (or maybe more) to get the level of security that you would require for very high amounts of money.
In contrast, with 1 minute blocks, you would get about 3 minutes to get the minimum security level (which is lower than the minimum in the previous case), maybe 10 minutes to get a reasonable security, and also about an hour to get the security that millionary transactions would require.
So, I don't think that making faster blocks would make much a difference. For instant transactions, I think we should wait until something like lightning network or bolt payment channels is available.
It currently takes that long to generate a zk proof but newer software and hardware will improve upon that. Discounting the use of transparent transactions for their lack of innovation over all the other networks is short sighted. They're reasonably quick and, through a third party like Trezor, can be done without the need to maintain a personal copy of the blockchain on the device being used to do the transaction - and then the recipient can perform their own shielded transaction if that's important to them.
The one minute block time is an interesting question but it's worth noting that some networks, like Dash, have alternate solutions for instant transactions that are already tested and in use.