Things like mass A/V updates, Windows Update downloads/installs, etc are bound to cause performance issues if the typical performance footprint is near a potential bottleneck. Most recently for one of my environments, extremely high storage latency was encountered from the automatic quick scan that is initiated when definations are updated, because every VM was issuing a high number of read OPS at the same time, which caused many of the guest OSes to stop responding on the network briefly because they couldn't access their disks quickly enough. It wasn't a networking issue at all, it was just the storage not being able to keep up.
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