Bootstrap 4 without all the clutter, including only the responsive grid, form and button styles.
Bootstrap 4 is such a large and feature-packed style framework that you don’t need every single one of the style rules. Whilst the whole file is small in the whole scheme of things, if you’re chasing page speed scores then cutting 40-60 KB can certainly be beneficial.
For a simple website that just has a form and typography then using this will help cut page load times.
Full Bootstrap 4.3.1 is 153 KB whilst this grid, form and button styling is just 92, this gets cut down even more with just the form styling:
Or form with the responsive grid styling: