Using Dark Reader might make certain sites load a little more slowly than usual, as the extension is processing and changing a site's original colors into dark mode.
Gmail, in particular, lags quite a bit whenever I go back to my Gmail tabs. Luckily, you can set Dark Reader to disable itself for certain sites that you visit often where you don't want any lag.
Also, you may occasionally get a bright white flash while visiting a new site or opening a tab. That's because, according to Dark Reader's FAQ, your web browser initially loads a site in the default color theme, which is usually bright. You can fix the bright flash by using a dark theme for your browser.