• Does each thread receive an individual process time slice, or are all threads sharing a single process time slice?

  Each thread recieves it's own timeslice.

• Can threads from the same process be distributed simultaneously onto different processor cores?

  Ambiguously worded, but any thread of a process can (and will without extraordinary measures to prevent it) be scheduled on any available core. And different threads from the same process may be running on different cores concurrently.

• How does thread creation overhead compare with vfork/execl overhead?

  It depends. On many things.

  Threads can have a heavy spawn-point cost because they do their data copying up front.

  Forks can appear cheaper up front because of COW, but without extreme care even apparently read-only references to COW pages can cause ongoing piecemeal duplication of data which can ultimately add up to a higher overhead.

All that said, you're asking the wrong questions. In most cases, the choice as to whether threads or forks are appropriate for a specific application are less about how fast you can spawn another.

It really depends upon what your application is doing. Does it need to share data bi-directionally? Does it require a spawn and discard approach or would it be better served with pooling?