There is a multithreaded NTP server written in Rust I was playing with some time ago: https://github.com/mlichvar/rsntp
On a VMware guest I have in the pool the maximum throughput improved from about 55kpps to 110kpps. The average rate is about 2kpps (with the maximum speed setting), so it only made a difference for spikes happening on midnight, like this one: https://i.imgur.com/MxsJt5F.png (rsntp wasn’t running here, so it responded only to about 55kpps).
Please note that the CPU utilization doesn’t scale linearly with traffic. With interrupt coalescing the kernel can process multiple packets per interrupt, so an NTP server should be able to handle more than double of what it can handle at 50% utilization, even if it is single-threaded.