Shinigami, it appears that we are talking about two different things entirely!
I had in mind a command queue: each thread of each survivor pops an opcode off the queue and pushes its next one and state to the bottom (like in RedCode). This seemed to be a reasonable way to implement "slowing". Twice as many threads means that the queue (and waiting time for each thread) is doubled.
In this implementation, if a thread dies during its turn it simply goes off the queue.
What you thought of means putting it in a "dead state" where it does nothing.
The way I saw it the more threads, the slower each one is, but the less threads, the faster each one is – the queue gets cleared as they are killed.
It appears that we had a misunderstanding.