You are currently only earning guild rep from questing. Based on this, your rep gain speed will be slow since you are missing out on the other two ways to earn it. (rated BG’s and boss kills) We just activated guild xp on dungeons so you should see the rate increase from that. You will see another increase once rated BG’s are active. Guild achievements will not affect guild rep since the xp that you earn goes directly to the guild and not the players.

We are watching the rep gain rate both internally and on the beta and we will tune as necessary right up to launch. Guild rep gain is scaled based on the amount of time we think a player should need to invest in a guild to earn the rewards and contribute. It’s not something that you can just “grind out” like a normal world faction. (Source)