The first thing to say is that "For a lawyer's office" is a fronting adverbial phrase. "As a lawyer's office" is not so idiomatic.
Next, there is an indirect comparison with what would be expected:
"For a lawyer's office, things are slow as shit down there." = "Compared to the typical lawyer's office, things are slow as shit down there."
OED gives
For:
VI. Of attributed or assumed character; = as.
19.a. In the character of, in the light of, as the/an equivalent to;
1719 D. Defoe Farther Adventures Robinson Crusoe 256 You'll be hanged for a Pirate.
1845 M. Pattison in Christian Remembrancer Jan. 68 Mere chronology..is often mistaken for history.
You will see that as also works, but does not work so well.
Whereas "as" introduces a similarity, for, in this context tends to introduce an example:
For (preposition) = As an example of
"For a small man, (small men are not often strong) you are very strong."