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Have a look at
Activity Monitor (
Applications / Utilities /) during playback and select
All Processes and sort by
CPU to see what the
culprit may be.
Also check the "
System Memory" tab to see what your "
Page ins:", "
Page outs:" and "
Swap used:" are.
image below uses sorting by CPU as an example
Image
Further reading:
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Did you also install the latest version of
Flash?
What video version (240p, 360p, 480p, 720p, 1080p) do you watch when lagging occurs?
What Mac OS X and Safari (or whatever browser you use) version do you run?