I have kind of noticed Monday mornings seem a little worse (at both counters and security checkpoints). Can't say reliably worse b/c I'm not there every Monday a.m. and can't say why generally. Tourists heading home? Gov't folks heading out on the road? Just don't know. One thing I have noticed is that TSA tends to get a little of a late start EVERY early morning at DCA and often (again, just as I recall) only opens one/two lines. Doesn't move very fast.
I will say I have never seen "short" AA lines at DCA. Nearly, every time I've flown AA out of DCA, I've been thankful for (a) status, (b) flying first class, (c) online checking (into a livable seat) and no checked bags. Where (a), (b) or (c) hasn't been an option, I silently weep.