There shouldn't be any 100k hard limit for threads at least in Linux, though you need enough memory for 100k stacks of course. You need to increase some default limits for it though (https://stackoverflow.com/a/26190804)

Assuming a generous(?) 20 kB per thread in stack and other corresponding OS bookkeeping inforation you could have 1k threads in 20 MB, or 1M threads in 20 GB.

Doing 100 Hz timer wakeups and IOs concurrently in 100k threads makes 10 M wakeups/second, that takes a chunk of CPU independent of green / native threads choice. Performance vs kernel threads will depend on the green threads implementation.