It loads wp-block-library-theme-css on your <head> which is another file within WP core.
And if so, is this not bad for sitespeed to load another css file?
You can check how much of an effect it does to your site on page test websites (GTMetrix, Google PSI, Webpagetest.org). But it shouldn’t have that much of an effect especially of your site is well-optimized.
A wise man once said:
"Have you cleared your cache?"