Neither of the files listed are very large, so they shouldn't take long to load, and shouldn't really affect load time that much.

The second file - content.css - is optional and is only required if you are using the JCE Pro Columns tool in your content. You can disable the loading of this file in the System - JCE plugin parameters.

The first file - jcemediabox.min.css - is required by MediaBox.

I ran the same test on yhe JCE site, which loads both those files (amongst others) and I got a significantly better result than you - https://gtmetrix.com/reports/www.joomlacontenteditor.net/zmEeB5H9/ - which is I suspect because I am using Cloudflare on the site, which applies additional caching and compression to the site.