Why?
The majority of Squirrelly's code is meant to deal with compiling a template. If you precompile your templates, you only need the code that deals with defining helpers, filters, partials, etc.
The runtime version is only really relevant in the browser, where load times are very important.
How?
Instead of including the full Squirrelly JS file, include:
<script src="https://unpkg.com/squirrelly@7/dist/squirrelly.runtime.min.js"></script>