No credit to myself here - all of that belongs to the author of this guide.
If you're not into eye candy boot screens and are comfortable moving around in a terminal then I recommend this guide.
My boot was about 25 and is now down to 20. From grub2 menu to Desktop.
If you take the black command windows as each step then be careful with steps 2 and 4
Step 2 is to disable the initramfs - if you are referencing your root with a UUID then you should change it to /dev/???? eg root=/dev/sda1. Because I was getting kernel panics until I did this.
Step 4 for removing services - if you want to keep your security ie firewall, selinux and sandbox you should probably skip this step and reference the page below instead and only disable the ones it recommends
It gives a good description of services and is still pretty current, but have a look at this other one for some other F15 specific services that can be disabled.
Anyways yeah I shaved about 5 seconds or more from my original boot time