Using REST API in WP Theme?

I’m consulting on a project where the previous developer used the REST API (instead of WP Queries and WP functions) to retrieve posts and then display them with Javascript loops. After a little troubleshooting, I have everything working functionally speaking, but the site seems to get pretty low page speed scores.

I have used the REST API many times for querying external sites, but never within a theme to retrieve posts from that same site’s db.

Is there any reason not to use the REST API from within a custom theme to query your own database or is it perfectly fine?