An interesting concept to consider is that the fastest website could actually be an empty webpage. Each additional piece of data added compromises performance, yet it's the server's responsibility to send all the code to the client.
When using bootstrap or any CSS library in general from a CDN, all the processing takes place on the client side after fetching the stylesheet. This can lead to significant delays.
Therefore, by including the complete bootstrap stylesheet (which is quite large) and placing it in a <style> tag on my Single Page App, most of the load would be shifted to the server. The impact on the server would be minimal compared to the load on the client since it is just plain text.
Could it be efficient enough to include all CSS code directly in the page itself?