Is it all emails or just particular ones that take longer then normal (say 15 minutes).
Are you sure the emails arrive to your mailbox with such delay or just get synced to K9 with such long intervals. Would be interesting to see. For example check via webmail if the emails are already there (in your mailbox) that would point into K9 being set to pull emails on such long intervals.
I did quick check in the logs but it doesn't look like anything out of ordinary. Also in case of some of the emails we run what's called greylisting which prevents instant delivery of emails originating from sources that are not frequent enough. This prevents most of the spam we are getting as those do not re-send emails as it's too 'expensive' for their operation (they push as much mail as possible before they are spotted and blocked). So in case of greylisting the delay might be around 15 minutes or so, but it actually depends on the setting on the other side (what is the server setting to re-send rejected emails). It's good to write to support_at_disroot.org if there is a particular email origin that does not get resend or if it indeed takes very long to receive such email so we can put it on the whitelist to prevent this in the future.