I personally think Zcash team is trying to solve a non-existing problem! Slow start would be helpful if very few people know about this project at launch but in fact Zcash has been already hyped. Just take a look at GPU miner thread.
Almost everyone invested in Bitcoin have already been informed about Zcash. That’s why mining slow start wouldn’t do anything beside adding unnecessary complexity to codebase. More importantly, discouraging honest miners from early start could give malicious parties more chance to dominate network and launch an attack against the blockchain.
If we want to avoid “instamine”, a fast difficulty adjustment like what we are testing now would simply do it.