Google provides a tool called PageSpeed Insights which gives a website some metrics to assess how well it is put together and how fast it loads. There are a lot of technical details but in general green scores are good, orange not great and red is bad.
I tried to ensure the tests were similar for each platform by choosing a page that shows a list of posts, like https://mastodon.social/explore.
PieFed and kbin do very well. pixelfed is pretty good, especially considering the image-heavy nature of the content.
The rest don’t seem to have prioritized performance or chose a software architecture that cannot be made to perform well on these metrics. It will be very interesting to see how that affects the cost of running large instances and the longevity of the platforms. Time will tell.