MAKEHASTESLOWLY (22A: Oxymoron for cautious travel) my foot! It seemed like a pretty fast solve for a Wednesday, but after my troubles yesterday, I'll take it.
ASTHICKASABRICK (50A: Simile for denseness) makes us think of the classic Jethro Tull album (although he didn't use the first "as"). So what is it, just a figure of speech theme? Hyperbole, Oxymoron, Litotes, and Simile? The only one that people might not know for sure, I'm guessing, is litotes, but who knows… maybe I overestimate people. Or underestimate them.
GALLEONS (4D: Many Spanish Armada ships) is lovely. And speaking of overestimating people, you might have guessed I'd know whence "Ave Maria," but no, I learned it from this puzzle. OTELLO (12D: Opera with "Ave Maria").
I didn't love KUE (55D: Scrabble 10-pointer, spelled out). I think "cue" Googles a bit better (in my ten-second search), but I wanted "kew" instead, but then, that was already in the grid… KEW (52D: London's ____ Gardens). KUE sounds more European, where they don't have that diphthong-y sound, but say it more like "coo." Oh well. NENE (56A: Leakes of reality TV) was a tough clue I'd never seen for that before, nor was I familiar with ADELA (21A: Writer ____ Rogers St. Johns), or RITT (16A: "Nuts" director Martin), or LUKAS (6D: "Last Days" actor Haas), or UTHER (46D: King Arthur's father ____ Pendragon).
I guess four fifteens is a decent amount of fill, and the phrases are all decent, so even though the fill is a bit strained (TRITEST, EELER, ANIS), and it's packed with obscure names, I still didn't really think it was terrible. Let's call it a wash.