Default Downgrade to older bootloader / rom

I think that my nexus 7 became more and more slow when installing new android updates. So I want to give it a try and flash a 4.1 rom, just for curiosity.. I can find older roms but I can't find older version of bootloaders. Is there any dedicated thread with older downloadable version of bootloaders? Can I extract those from factory images?
So far I learned that to downgrade I have only to flash an older bootloader and then the older rom. Is that correct? Thanks for the help.

Inviato dal mio Nexus 5 utilizzando Tapatalk
Sorry for my english