Time to build has two issues. If quality of work is the focus, then spending time to make SURE things like concrete aren't deficient in some way. Going with cheap materials might cause problems later. Being extra careful with quality is just a good idea. And then, you have the lazy contractor issue, where some politicians don't impose strict guidelines and rules. If the contractor claims it will be done in 2 years for XXXXX amount of money, then government should say, "if it takes 10 years, you won't get additional money just because you were goofing off. The only reason for timeline extensions should be if there are good excuses for the delays in completing the project, such as, "we had a huge storm that flooded the site, and it took six months or longer to get things cleaned up so work can continue"
If contractors couldn't get additional money because they take longer than their estimates, they might actually complete projects faster than 25 years.