Time Machine backup extremely slow after upgrade to Yosemite
I have a Macbook air late 2013 on Yosemite with similar specs but I have an Apple time capsule instead and the only thing I have found to improve speed is an ethernet cable. Get a connecter from thunderbolt to ethernet and then use an ethernet cable to connect it to the external drive. I haven't really tested this much but it did work the few times I have. Also, your external drive might be getting old. For me it usually takes about an hour or less to do a 90 GB backup. Also check your other computers or devices because if they are backing up, it will slow it down. Hopefully this helps!