Is it inefficient to repeat a Shortcode 100 times on a page?

For Non-WordPress Person: It solely depends on what plugin are you using or what theme are you using. There are a lot of tools to check whether your page is doing well on the internet or not. One of them I use is GTmetrix. Enter your page URL and it will give you brief report. Good news is its totally FREE. There you can see what is the page load time and how many requests your page is doing. You should always try to reduce external requests and then internal like images. If your page loads below 4s is good sign that even if you have more than 50 requests, you site still perform better.

For WordPress Professional Person: If you are WordPress Developer and try to dig into deep then my suggestion to you to use REST API. And try to load more Friends on a scroll. This way you load only a few of them first and try to load rest on a scroll.

I hope it will help.