I'm surprised that more people don't transit through SJU. I often hang out there for a few days either coming or going. Plus, no matter where home is, flights from home to SJU are often cheaper than flights from home to SXM. Immigration in SJU is usually a breeze, as long as you don't come in behind a big plane. Immigration has now moved to the A Concourse. They have one or two Global Entry kiosks, and 10-12 regular kiosks. I've gone through there when I am the entire line. There's a Pre-Check line at security. If you have a Priority Pass membership you can access the Avianca Lounge between terminals B and C.