I've built some websites for local businesses (general 5-10 pages) using WP

But now I'm really getting strict about load times. It seems that using Grav would be faster. A lot of Grav-based sites that I've seen are blogs and portfolios. Would Grav be suitable for a general small business website?

Grav is a general purpose CMS. You can have any kind of content and organization you like.

Take a look what GRAV can do:

here
and
here

I had very good experiences with grav as an cms for small businesses. It's much simpler than WP and even easier to deploy and secure because you don't have any database to wrangle with ;)

If you need help implementing things I'm more than happy to help. You can try to send me a PM here (though that failed for me the last time here :D )