You can read about its IO performance against QEMU in their NSDI paper (table 8,9): https://www.usenix.org/system/files/nsdi20-paper-agache.pdf

Even though the startup times are fast, the IO performance is poor compared to native. This is primarily because of IO emulation. We've been working on a new hypervisor that can directly run isolated containers (no VMs). Email me if you are interested in learning more.