The `JvmtiVTMSTransitionDisabler`sync protocol is on a performance critical path of the virtual threads mount state transitions (VTMS transitions). It has a noticeable performance overhead (about 10%) which contributes to the combined JVMTI performance overhead when Java apps are executed with loaded JVMTI agents.

Also, please, see another/related performance issue (contributes ~70% of total performance overhead):
JDK-8308614: Enabling JVMTI ClassLoad event slows down vthread creation by factor 10