Is this a non-Meraki VPN (as opposed to AutoVPN)? If so, you might be experiencing an MTU squeeze. You could test this by reducing the MTU on one workstation to something like 1300 and see if it resolves it. If it does resolve it, open a case with support and ask them to reduce the MSS on the MX to 1300. https://support.zen.co.uk/kb/Knowledgebase/Changing-the-MTU-size-in-Windows-Vista-7-or-8 Failing that, you are going to need to take packet captures to figure out what is going wrong.
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