I can't use all link capacity using tokio

[HKalbasi]

I have a 20Gbit/s link. I open 5-50 tcp connections with tokio and the server is a simple echo server. When I write plain data in the connection, it easily reaches the link capacity, but when I use tls using tokio-rustls crate, it max at ~16Gbit/s. My cpu cores are 30-50%, and overall cpu usage is 800% (I have 24 cores) so cpu is not the bottleneck. And when I run two of those client processes, they can max the link together (15Gbit the first one and 5 Gbit the second one), so I think there is a problem/misconfiguration related to tokio happening here.

Do you know why this happen and how I can fix it?