The Comptes Rendus is (are) a French Academy of Sciences publication, essentially a 'rapid communication' format. It comes in many flavours, and maths is one of them. Proofs are non-existent (at least in older articles I've read), or perhaps only sketched. I don't know what the reach of CR is outside the francophone world (note that CR is bilingual French/English)
One preliminary question is this:
Are there any equivalents to CR?
Some context: say I have a new result, which I know will be true, and just the grinding calculations need to be done. If I don't want to wait to unveil it to the world, what benefit do I get from publishing in rapid communication journals like CR? I could put the same thing on the arXiv, and while one might point out that a journal is refereed, but how can a proofless/proof-lite article be refereed?
Should I submit to a 'rapid communication' style journal while I finish the work, and then submit the full article elsewhere (+post on arXiv), or can I get away with a short arXiv note and then post and submit the full version when it is done?