If you need to add HTML, this can be done in a theme file. Which files depends on where you need to add your HTML.
Usually such code would be added either just after the <body> tag which would be in header.php, or at the end before </body> which would be in footer.php.
You could edit these files from Appearance > Editor but this is strongly discouraged: if you edit a file from the theme directly, any future update of the theme will get rid of your changes because updates overwrite every file.
Tthe proper way to avoid this is to create a child theme, then copy the file you want to edit in the child theme and edit this copy.
If I may make a suggestion however a spinning loader is probably not the best solution for your loading time problem: it would be actually one more thing to load so the overall load time would become even longer, and the loader itself may not even load and show instantly.
A better approach IMO would be to work on making your site load faster.
How to do this is way beyond the scope of theme support but to get you started the first quick steps with high-results I would recommend are:
– Enable gzip compression. This alone can reduce your load time by 50%. Here’s quick tutorial I wrote: https://www.iceablethemes.com/how-to-speed-up-your-wordpress-site-with-gzip-compression/
– Use a caching plugin. Among the most popular are WP super cache and W3 total cache. Pick one (never install both at the same time!) and take a few minutes to make sure to tweak the settings – google it for info and tutorials – and it will also make quite a difference.
