Horrendous Pagespeed results

We are a smallish company in the UK midlands, with a commercial website, but is slow. Recent Google PageSpeed runs have shown a marked reduction from the 90’s to 20’s.
I believe we need a consultant familiar with both software web performance, and also Windows Server, IIS, Umbraco and VMware to basically find the performance failings.

In case you can help this is our setup;
Dev team of 3, .Net website using Umbraco 8, IIS, MS SQL

Cisco ASA firewall, VMware Host, pair of Linux ha proxy load balancers, pair of Windows Server 2016 web servers, pair of Windows Server 2016 and SQL Server 2016 Std in Always on AG (active/passive)