It would seem that your app has only 2 threads, hence the relative 50% limit. Be careful with 4 vCPU guests, as they can cause CPU scheduling delays on many hosts. In general the recommendation is to allocate a maximum of half the number of physical processor cores to any one guest, but if there were two such guests there would be no free CPU resource for underlying ESX functions (for example iSCSI, NFS, memory management etc etc).

http://blog.peacon.co.uk

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Unofficial List of USB Passthrough Working Devices