While in the debugger I noticed a large number of calls to nsHTMLMediaElement::ChangeReadyState via UpdateReadyStateForData hitting the early-exit path because the ready state was not changing. MediaDecoderStateMachine::UpdateReadyState is called from multiple places and dispatches an event each time, even if the state is unchanged. This can result in many events being created and dispatched during a single video frame. With a simple current == last status early-exit path added to MediaDecoderStateMachine::UpdateReadyState, the number of events created and dispatched for a simple play-through of a 5 minute video drops from ~140,000 (~468/s or ~15/frame) to 3.

Tests pass locally. Try push: https://tbpl.mozilla.org/?tree=Try&rev=9c79c64b62d2

Comment on attachment 693745 [details] [diff] [review] patch v0 Review of attachment 693745 [details] [diff] [review]: ----------------------------------------------------------------- Good catch