The performance of their first showcase site is extremely slow. Svelte's fault? Even if the API is slow behind it, why would it block the rendering of some content? https://www.attendzen.io/
I work at AttendZen -- our product is built using Svelte, but our marketing website (currently) isn't. I'll investigate to see what may have caused a slowdown here, but it's definitely not down to Svelte.
It is pretty fast for me in central Europe. Maybe it's latency?