Very slow start-up after Yosemite upgrade

My MacBook Pro (13-inch early 2011) has been behaving slowly for a while. At first I thought this was just because my computer is several years old. But, I also realized that it has become progressively worse with each OS X upgrade. Now that I have installed Yosemite, it's the slowest it has ever been. I upgraded the RAM from 4GB to 8GB and, while this has sped up performance while my computer is on, the start-up still takes a really long time. Once I turn my laptop on, the gray screen stays on for several minutes and the progress bar gets stuck at the half mark for a while. Meanwhile, other Macs that I work with start up in 2 seconds. I've removed the contents of the StartupItems folder in the Library (as some threads have suggested) with no result.

Any solutions?

Thank you in advance,

Paula