I would suggest reviewing the Nimble Best Practices for iSCSI located within the Support site under the Best Practices Tab. Ensure that Nimble, the network ports serving iSCSI, the Host NICs are all configured correctly. Also, ensure the switch has available buffering to service the IO query. Lastly, check that MPIO is set correctly on the host and ensure the Nimble toolkit is installed so that the Windows Registry settings for iSCSI are adjusted as the default are defined minimally. the toolkit will increase these values
In addition, I would also read through the Recommended Windows Server Updates article on InfoSight and make sure that all the updates are applied: https://infosight.nimblestorage.com/InfoSight/static/extras/Nimble_NCM_NWT_WINDOWS_HOTFIXES.pdf.
If all else fails, lob a quick call into Support and see if they provide more insight.