The short visits are due to accessibility (incompatible with a browser or too long to load) and/or the content. Some designs/layouts are so poor, that people are repulsed, but I'm going to assume that is not the case here. As for design, a person should be able to simply glance at a Home page and have a general idea of what the site has as content/theme.

30 seconds is a fairly long time. Hopefully, that is not spent waiting for the page to load. If they are trying to navigate and don't get the feeling that they are at leasting closing in on what they want, they will leave.

I think we's all like to have a look at the site to give you a better answer.

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DrTandem's San Diego Web Page Design, drtandem.com