High-assurance isn't really the culprit. It's more of a funding issue. If long NASA programs could save part of the first year's budget to use the second year, things would go a lot faster.

You might get hired into an overstaffed team and not do that much for a year until it's crunch time and then you're underfunded so it takes an extra year. Arguably you spent three years to achieve one year's worth of accomplishments.