The configure script sets a define to enable the thread memory allocator if you enable threads. I disabled it by hacking the configure script:
# USE_THREAD_ALLOC tells us to try the special thread-based
# allocator that significantly reduces lock contention
# AC_DEFINE(USE_THREAD_ALLOC)
(There is now a # in front.)
If you do this TCL will use its normal memory manager, which is completely thread safe. I notice no performance difference, allthough my site has rather low traffic. It saves a lot of memory.