Sql Server 2012 Failover Replica in Synchronous Mode between Primary-Secondary on VPN is very slow
Because you're using Synchronous replication, all transactions have to be signed off by both servers before your query can complete. You say you have a 'high speed' VPN, but, you don't make any mention of latency or bandwidth.

I suspect that you'll never be happy with the performance of synchronous replication, due to the additional latency. You should strongly consider setting the HA replication to Asynchronous.

Source: https://msdn.microsoft.com/en-us/library/ff877931.aspx