Parse and enforce cosmetic filters. Enabling this feature increases uBlock₀'s memory footprint.

Selecting this option still results in cosmetic filters being applied late in the page load, resulting in flickering which tends to be pretty drastic shifts in page layouts.

I assume this is the trade-off you're willing to make to avoid the overhead of injecting the stylesheet as a straight up userscript which is understandable, but I don't think this trade-off is necessary since there's already a better method of applying stylesheets more efficiently.

For the better part of the last year, the Stylish extension has been improved to apply stylesheets virtually seamlessly, arguably faster and less glitchy than a userscript and without the major memory drain.

Hopefully this method could be incorporated in Ublock:
https://github.com/JasonBarnabe/stylish-chrome/blob/master/apply.js