I think that the implementation time is obviously a knock on the administration and Congress. The bill would have obviously been better structured as a tax-break and jobs creation bill that had several phases. However, this Congress was not interested (particularly in the House) in offering tax breaks. They wanted projects and an almost purely Keynesian spending bill.
While I think that spending and jobs creation can certainly work, they take time - and tax breaks would have obviously been a quicker way to get some of the money into the economy.
I think that something that hasn't been brought up yet, but is important, is the nature of the administration when this bill was passed. I talked to the CFO of the DOE after the bill had passed, and he told me that he and the secretary alone were the only two high-ranking officials in the department (and this was quite some time after the election...probably in April) who were actually on the job (he was one of the very few holdovers from the previous administration). The nomination/confirmation process takes forever and most of the agencies were left short-handed at the ranks where these types of high-level decisions are made. He was finding himself swamped in trying to make sure the money got out the door...while at the same time trying to be careful to make sure the money was not going to be wasted.
While the career service employees were still around, things needed to be signed off on and new procedures were being put in place...really bogging a lot of things down.
Trying to get a 1 trillion dollar spending bill going while at the same time having a complete administration overhaul (because of the party change) is going to lead to some serious delays, which I think are a big part of why only (ha ha) 100 billion dollars of the money has been spent.