Reset locked iPhone 5 without updating
Afraid not, but I feel your struggle. My internet is about as terrible as it gets (14kb/s). However, I was able to download the IPSW using my PS3's Internet browser which seems to get 700kb/s and it took slightly over one hour. You might not have a PS3 but this is what worked for me. 

Or likewise, you could try downloading the IPSW on a public library computer from a 3rd party website or even a friend's computer. Whatever the case, good luck.