After trying various things to improve the FPS I came across the following registry tweak which changed my FPS from 20 to 27. If you try it, make a note of the settings before you change them, and reboot.
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Multimedia\SystemProfile\Tasks\Games]
"GPU Priority"=dword:00000001
"Latency Sensitive"="True"
"Priority"=dword:00000001
"Scheduling Category"="High"
"SFIO Priority"="High"