20 minutes for all bar 17 and 24 then another 11 on these. Mostly very easy, mais il y a une mauvaise surprise, non?
Across
1 S+CAMP
4 BIG DIPPER - switchback is another term for a roller coaster.
9 OBSTACLES - classtobe*
10 P[ARK]A - among other things, the Ark of the Covenant, before Harrison Ford and the Nazis got their hands on it, contained the Ten Commandments and Aaron's Rod (the one that blossoms - sometimes in crossword puzzles).
11 JUST SO [S]TORIES
14 MIEN - sounds like 'mean', although I'm not sure that that's how I've ever perceived the word, giving it a faux frenchified internalised pronunciation, internalised as I'm not sure I've ever essayed the word in public. Fortunately.
15 ARISTOCRAT - isactor* in ART.
18 EXPOSITION - très facile, non?
19 M+ILL - encore, bien sur.
21 DRILL SERGEANT - as well as being a baboon with a short tail, drill also passes muster as a coarse twilled cotton or linen fabric.
24 EVITA - ETA containing V (against) and I (one); nice clue, which requires ordinary folk to lift and separate 'pressing against', while speedsters will chuck EVITA straight in from the initial e.
25 CHEONGSAM - mochanges*
27 omitted - don't wish to boar you with this one. On edit: since it's causing a bit of confusion, it's PIG-HEADED, where the setter draws on a usage common in horse-racing parlance, 'Lucky Dragon was headed in the shadow of the post'.
28 A[BY]SS - I never actually looked at 2 when solving.
Down
1 SHOWJUMPER - show as in "show/lead them in, please"; jumper as in thing you wear as a top, or maybe on top.
2 AS=S[tate]
3 PRAISE - P[ressure] + RAISE; but I can't see how cause = raise. On edit: Jack's found it in Collins and the example clinches it: 'to give rise to; cause or provoke ⇒ to raise a smile'.
4 BEL ESPRIT - B[orn] + reptiles*; more French: not an expression I was familiar with, it means a person full of wit/ingenium, hence a genius.
5 omitted
6 IMPRISON - MP in IRIS + O[ver] (our [French?] cricketing mot du jour) + N[arcotics].
7 PERESTROIKA - peers* + TROIKA; my only concern was whether it/they was/were spelt with a k or a c.
8 omitted - even easier than the two others omitted thus far, which is no mean feat.
12 SUET PUDDING - undisputed* + G; I'll leave our senior members, from Yorkshire and Barsetshire-via-Battersea, to comment on the taste of these things made from the white fat on the kidneys and loins of cattle and sheep, as I'm much too young ever to have tasted, or even seen, one.
13 AT ALL TIMES - 'a tall Times', where remarkable means 'fanciful and difficult to believe', which seems spot on to me.
16 STONE DEAD - ONE (a person) + D[eparts] inside STEAD (place).
17 ASPIRATE - AS (like) + PIRATE (to copy illegally); the literal is 'a Parisian doesn't'. Ma dernière entrée. Many an acoustic phonetician will tell you that the difference between 'dock' and 'tock', for instance, is as much to do with aspiration as with voicing.
20 AG[END]A
22 LUCID - it is.
23 R[E]AP - 'voluble talk' for rap is good. If I had been setting the puzzle (be thankful for small mercies), the clue might need to have been transferred to Private Eye.
26 omitted