How to fix delivery delays in Email Routing Beta

I am using the Email Routing Beta to direct emails to a Gmail account and nearly all emails seem to arrive within a few moments. However, I am finding that emails from one particular domain seem to be arriving with anywhere from a 6 to 36 hour delay. I would welcome any suggestions for troubleshooting this!

How long do emails from that particular domain take to arrive if sent directly to your Gmail address?