Not the OP, but my experience matches his and for me the following are particularly important:

- Getting the requirements right. This by far has the most impact on effectiveness

- Reducing friction. Simple things like proficiency with high quality tools, smooth and fast builds, clean logs, easy ways to generate and analyze core dumps, accumulate to produce an outsize effect on productivity.

- Uninterrupted, substantial blocks of focused time. It is staggering how easily chat, email and meetings can destroy productivity.