In something like this:

.ui[class*="very basic"].table:not(.sortable):not(.striped) td:first-child, .ui[class*="very basic"].table:not(.sortable):not(.striped) th:first-child

in many editors I receive this warning:

*Don't use adjoining classes. Attribute selectors with = are slow!

Is this true?

How this can impact in my project performances?