In the novel Dark Lord: The Rise of Darth Vader Tarkin says that the first Death Star is experiencing multiple delays due to not enough workers, contractors not fullfiling their deals, supply problems, and other things. If this was the case then I am sure the first Death Star took longer than the second. Also, remember that the senate was still around when the first one was being built and the emperor was trying to hide it from them, and this hindered who he could hire for fear of the wrong people knowing and bad publicity. By the time the second one was being built the senate was disbanded and he could hire anyone he wanted and basically do whatever he wanted. The emperor was using basically the entire imperial force and power to build the second one so this also spead up the process. So I think the timeframes were realistic.