DOCFX Build is extremely slow - times out 1 hour limit for AppVeyor builds

**Operating System**: `Windows`

**DocFX Version Used**: 2.42.2

**Template used**: custom (based on statictoc)

**Steps to Reproduce**:
1. clone the repo from github https://github.com/UbiquityDotNET/Llvm.NET/tree/f717300813a11f6bf9621dc740e7772cd783b08d (This commit point shows the problem and won't include any workarounds I end up with for dealing with this)
2. Open powershell command prompt to the newly cloned source location
3. run .\Buildall.ps1

**Expected Behavior**:
All code and docs build in a reasonable amount of time.

**Actual Behavior**:
Total build time exceeds 1 hour on AppVeyor builds causing a failure due to time limits of 1 hour per job. Overwhelming majority of the total time is all in the DOCFX stage. (see: [AppVeyor logs](https://ci.appveyor.com/project/UbiquityDotNet/llvm-net/builds/26017197) of failing build for more details)