alt Hacker NewsYes, it's not a new idea. See Microsoft Singularity research OS for a more recent example.
You would still have slower than native execution speed, security problems with Spectre attacks (in HW process isolation it's avoided by not having other memory mapped at all, other mitigations exist but they cost a lot of performance).
Syscall speedups affect only specific programs or stupidly written SW like some games that are checking current time many many times per frame.
It's not just syscalls. If you trust all of the running code, you can use a single address space for all applications and even the kernel, which lets you avoid TLB flushes.
Whether that's still a good idea given Spectre is another question.