The Custom Check Interval seems to have a minimal limit to '1 minute'.
Do not mix up "Push notifications" which comes up within seconds after a new message arrived to the server (= your mail service provider) and AquaMail's "Schedule sync" (mail check interval).
Synchronisation means: make the changes that occurred on server side (delete, update, reply, star, read, etc.) available on device (client) side, and conversely. Sync is a scheduled process (event based and/or time based). I strongly recommend leaving this setting at the default (every 15 minutes): less battery drain and lower data usage!
So Push and Sync are not the same thing. The great advantage of AquaMail is to propose them separately, up to you to select one of them or both.