The downside is that saving and retrieving data takes a bit longer when accessing a ZIP drive than a hard drive. I thought this stuff traveled at 186,000 mile per second. As pilots who land fighter jets on the decks of aircraft carriers like to point out, going real fast isn't anywhere near the challenge as is landing in one piece. One packet might head from Chicago to Seattle by way of San Francisco while another bit of the same message might go from Chicago to Atlanta to Tokyo to Seattle. The dozens or even hundreds or thousands of IP packets that hold your message move about the network until they reach the computer called an e-mail server used by whomever you are addressing.
Delays come from all sorts of things.
Your own server probably waits until a fairly sizable bunch of mail is queued up before sending it all off. At the other end, the server must wait until all of the scattered bits of each incoming message are collected before reconstituting the note and storing it for later delivery. Many a doctor has lost a patient by advising someone with a bad headache to take an aspirin and call back in the morning. Keep that in mind if troubles erupt a bit down the road. Meanwhile, you can fix the problem by the electronic equivalent of taking an aspirin.