IIS - RPS suddendly drops to 0 - threads blocked on releasing handles

This is the scenario: an IIS� web site serving files from a local home directory. Thing is, when I test it (through a VS performance test), at some point RPS (request per sec) drops to 0 from, say, 1000, comes back up, then drops, etc.

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While RPS is in 0, disk I/O in that same drive seems very slow (e.g. something like dir /s simply stops working). Also, debugger shows that most threads are blocked on releasing handles:

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Code Snippet
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