Does promotion in the Guard come slow?

It all depends on your MOS, state and number of slots for that rank.

In my area, it seems people stay put more often in the guard so there is less movement and less opportunity for a slot to open up for promotion. In the reserves most just take the promotion to another unit that has an open slot and stay the required time. I know I was a 92L made the SFC list for promotion and unfortunately there are very few E-7 Lima slots. I just got back from a tour in Afghanistan and my unit had the only slot within 300 miles but was deploying in 3 months so HQ promoted me down the hall as a 92F, I decided to retire than stay the required time in a unit I did not like.

It does or less you are in the right place at the right time