Why do you think so? For compute (non syscall bound) it should be native speed (just like a VM). For syscall-heavy operations it will depend on the syscall and how it's implemented and the backend processing.
For example, if you call lots of syscalls that are fully implemented in gVisor (does not rely o an external backend/service for completing them) that happen to be implemented inefficiently in gVisor (because they aren't top priority right now/not a lot of users rely on them being fast) vs the same syscall being already optimized in Linux then obviously there will be a lot of difference. But if calling syscalls which need an external service (ex. network/storage) to complete the request, then depending on the latency of the external service the processing speed difference of gVisor vs guest Linux kernel may not matter.
It really depends on the workload, there is indeed potential that some workloads will be significantly slower with gVisor, all other things being equal, but it doesn't seem to me to be a general thing.
For example, if you call lots of syscalls that are fully implemented in gVisor (does not rely o an external backend/service for completing them) that happen to be implemented inefficiently in gVisor (because they aren't top priority right now/not a lot of users rely on them being fast) vs the same syscall being already optimized in Linux then obviously there will be a lot of difference. But if calling syscalls which need an external service (ex. network/storage) to complete the request, then depending on the latency of the external service the processing speed difference of gVisor vs guest Linux kernel may not matter.
It really depends on the workload, there is indeed potential that some workloads will be significantly slower with gVisor, all other things being equal, but it doesn't seem to me to be a general thing.