I've been using unikraft (https://unikraft.org/) unikernels for a while and the startup times are quite impressive (easily sub-second for our Rust application).
Fast boot up means nothing if your agent/app is slow at runtime (due to virtualization tax or QEMU emulation). Fast boot up is a PR term, which can easily be optimized for compared to designed a better virtualization layer that performs near-bare-metal.
What drove you to choose that over something like containers?