I have a Huawei P30 and it periodically reports that my K9 has high battery usage. I can also confirm this in the settings under usage overview.
In my account settings under “Retrieve emails” (German translation, hope that’s correct), I have set “Frequency of email retrieval” to every 2 hours.
Then I just sent myself an email via the PC and another email account. After 1 minute it arrived at K9. Shortly afterwards I sent myself another e-mail and it also arrived directly.
But my expectation would have been that the mails are only checked every 2 hours.