Mac boots into log on screen quickly but takes long time to log into account
This is normal behaviour for Yosemite for users with FileVault enabled (i.e. Your Mac's primary disk is encrypted). 

The authentication process is done earlier because without authorisation from the user the OS cannot access the rest of the disk.

This is a good thing, and I think you are (mostly) mistaken that it takes longer to boot up — since now you are watching the screen and waiting after entering your login/password the result is that one fixates on the screen and the time it takes to present the OS X graphical user environment, and comparing it with your prior experience of how much time it took to go from authentication to the GU environment. But the total time from power up shouldn't be noticeably longer.

To disable this behaviour:

Open System Preferences > Security &amp; Privacy > FileVault

From there you can click on the 'Turn off FileVault', after which you will be asked to restart and you will have the normal boot up behaviour and the OS will start to decrypt your disk in the background.



EDIT: The 'extra long' boot time with FileVault occurs once only, at the first startup/restart after enabling FileVault as the OS is finishing the process required. After this, it should be as fast as normal. I double-checked this whole process by disabling FileVault and re-enabling it on one of my machines so I could be sure.

Nb. (if there is any slight increased length of time in booting up, it is more likely as a result of the disk encryption not because of your upgrade to Yosemite per se.)