fastcgi cpu usage on IIS 8.5

Hi All,
I’m trying to track down a performance problem with our new internal wordpress site. I’d appreciate some help.
Server 2012 R2, IIS 8.5
The minimum response time for any page seems to be 1.5 seconds – (3.5 seconds after an IISRESET) – regardless of page, plugins etc.
fastcgi we can see a small spike in CPU from the fastcgi process. However if I simulate some more load by refreshing a page very quickly I can max out the cpu for 20+ seconds with multiple fastcgi processes.

Disabling all plugins doesn’t impact performance greatly
I have caching enabled (wincache, W3 total cache)
I have enabled 32 bit applications and set start mode to always running in IIS default web site and app pool advanced properties

Using W3 total cache I have experimented with all different kinds of caching – nothing seems to make it any better.

I can see using fiddler that once the initial delay the page loads pretty quickly. I’m not sure I can go live with it in this state!

Any help greatly appreciated.