Mail.app and accountsd are taking op all CPU Mac OS 10.15
Same here, problem seems to be closed related to 10.15, since on Mojave I've never experienced something like that. Googling around, I found that the problem seems to be due to some "under-the-hood" logic behind account configuration.

Specifically I found that going to

Mail.app -> Settings -> Accounts -> (repeat for each account you have) -> Server Settings:

Disable the option: "Automatically Manage connection settings"

This seems to lower at 0.0 / 0.1 the CPU requirements for accountsd.