Well, This is like...The Only NON FLAME comment, But okay,
This is happening because sence the Relese of Steam for the mac, and free portal, And what not, Everything, and everyone (Including Striders!) are using the steam server. NOW to fix this, You need to...
1. Go to Steam Account Prefrences, Download Prefs, And Set your Network Connection to 10M Fiber Cable (Even If your not using it) Set your location to your closest.
2. Use TCP! TCP, Okay, On windows you can easily right click and set Launch options, But on a mac you can't so what you need to do is Open Terminal (Use Spotlight)
And type in "Open" then Drag Steam into terminal, Press spacebar then type "--Args -tcp"
If you do all of this, It SHOULD work, Also Steam for mac is not that great on a Laptop if thats what your using, Its best on an nVida Graphics Carded iMac, (All new iMacs have nVida's and all Mac Laptops have whats called a GMA which sucks at playing Steamgames)
I hoped this help, If it didn't Just wait for the Servers to calm down.