I don't think the "Don't use too many floats" is meant to say anything about performance, but about maintenance.
I think the point is, that if you have to put "float" in lots of declaration, you're missing some abstractions in your css. Most likely a grid plus some classes for inline lists.
Again, CSS Lint is very opinionated, and some of it opinions are contrary to various best practices (especially the fundamentalist approach to not let any classes in the HTML have anything to do with presentation). But from the perspective of these opinions based on how to write maintainable CSS and HTML for large sites, the rules makes a lot of sense.
I think the point is, that if you have to put "float" in lots of declaration, you're missing some abstractions in your css. Most likely a grid plus some classes for inline lists.
Again, CSS Lint is very opinionated, and some of it opinions are contrary to various best practices (especially the fundamentalist approach to not let any classes in the HTML have anything to do with presentation). But from the perspective of these opinions based on how to write maintainable CSS and HTML for large sites, the rules makes a lot of sense.