You are correct about the speed, but the short response combined with the unjustifiedly overly general attack of the authors knowledge is unnecessary.

Kernel code also uses virtual memory. There is no such thing as using "physical memory" unless you are running in real mode. CPU is switched to protected mode during boot.

It depends on what you're doing. Eliminating context switches can be hugely valuable. I agree the author should have been more nuanced, but, he's not flat-out wrong.

That's actually true. There might be some more initialization overhead if you exec a process (the libc stuff), but non-kernel (i.e. user space) code is not slower per se.