Do some profiling.
See for example ecma3/Date/e15_9_5_10, which does some simple arithmetic but takes forever on lightspark. It completets on the proprietary player in seconds.
I see this too. On my machine about 30 tests time out, the majority of them from the Date suite. Allowed to run they complete in about 20 seconds.
ecma3/Date/e15_9_5_10 takes about 36 seconds (24 seconds with '-j -ni') on my machine (Release build). The proprietary player runs this in under 2 seconds.
It looks like the most overhead comes from atomic_decrement as well as other threadsafety things.