My guess is that they are pretty casual. Our guild is casual and not focused on naxx. Now that we have a bunch (something 100) 80s we have people interested in doing naxx. We started last week and cleared the spider quarter. I just hit 80 on my hunter and hope to join them this week. This is my casual guild (horde).

My alliance guild is hard-core. They hit 80 with a few days of expansion, within two weeks they were doing naxx, within a month they cleared it with 25 ppl.

Really depends on the people I think. Many/most of the player base simply aren't wanting to be serious 3x a week raiders.

Your guild =/= every guild. Your raid schedule is not relevant. Not everyone is "plowing through the content". Some people are very happy to be in casual guilds that are just starting 10 man content.

That said, it sounds like the OP's guild is making strong progress, and if everyone's happy, it's a good thing. Hope he's not disappointed that life moves on without one person and doesn't get hung up on the ego thing. A lot can change for DPS between a month of gearing and patch changes. Make sure you're up to date on any changes for your class - eg. if you're still playing BM the way BM worked a month ago, no wonder your DPS has fallen. If your guildies have T7.5 and you're in heroic blues, no wonder you're not topping the meters.