When you use ithreads properly, you create the threads soon, when there are only very few data in the heap, and you use as few shared data as possible. When ithreads is used this way, it should be reasonably fast.

This means that the ithreads model is not ideal for some tasks you could normally do with threads. Ithreads are, however, very good for tasks where the threads do almost independent work, such as fork emulation or servers handling multiple connections. It is not suited for other tasks, such as tasks with heavy paralel computation imo.

Thus, I aggree with your original post in that ithreads might not always be what you want.

I do not know about any high level language supporting a thread model similar to for example pthreads, probably because that would make the interpreter unstable when you do not uses locks properly (5005Threads were like that iirc). If you need such threads, you migt want to look at other languages than Perl. Ruby has continuation-based threads, that means of course that it uses only one cpu at a time.