Ask HN: Why is the server waiting 10s to send TLS ServerHello?

Have a server app (cannot share it's name) where on Windows Server 2012 R2 it operates fine with ~1-2K TIME_WAIT / ~50 ESTABLISHED connections at any given time. Problem seen on Windows Server 2016 is it stays at ~5 TIME_WAIT / ~200 ESTABLISHED connections, but is rejecting lots of other connection attempts with immediate RSTs, also consistently seeing TLS handshakes with ~10s delays between ClientHello and ServerHello. We can't figure out what's causing the 10s delay - there is NOTHING between the client and server. 90% of the TLS connections are spent waiting on this delay, and so I think it's safe to assume 90% of the ~200 ESTABLISHED ones are in this delayed state. Any ideas?