Hello @ayham,
Thanks for asking.
I don’t think transferring modified CSS will make much of a difference to page load time. On the other hand I would suggest you to add CSS codes in child theme style.css file. As a webmaster our job is to also reduce page weight as its by far the most important factor in page load time. We do that by not adding unnecessary contents, codes, making sure the layout and code is clean etc.
Please note that there are bunch of other ways that can be used to significantly improve page load time like taking advantage of good quality hosting, caching plugin, CDN, compressing image file size without sacrificing quality etc. I would highly recommend you to refer our performance improvement guide to optimize page load time:
Thanks.