Are you using the OfflineIMAP SQLite driver? If not, the maintainers did say it was more performant and reliable than using text files which was the default before version 7.
I haven't dug into why it uses that CPU in any detail. Looking at its output / tcpdump of what it's doing it goes to >100% CPU for 10-30 seconds when it's finished fetching the status from the remote & is presumably in some tight inner loop comparing it with local data to decide what to do before it starts downloading messages, and then briefly spends a few more seconds at >100% after the messages are downloaded, presumably updating its local state.
When it's just issuing IMAP commands or downloading messages from the remote it sits at 1-5% CPU.
As I pointed out OfflineIMAP works just fine for me, but in retrospect I'd be happier if I had stayed with mbsync. I don't need any feature from OfflineIMAP that it doesn't have, and it both uses less CPU/Memory & it finishes deciding what to download & downloading it in way less time.
Thank you for mentioning mbsync, I will keep it in mind.