overrideHost script function causes increase in Doc Complete

When performing tests using the overrideHost header for scripting, it appears that the Doc Complete time increases depending on location that is being used. Two examples below:

https://www.safetydirect.ie/
Slow (Sydney, Australia - EC2 agent) - https://webpagetest.org/result/210212_DiKQ_db822c427da82c9a956fdb24eb35f65a/
Normal (Dulles, VA agent) - https://webpagetest.org/result/210212_DiPD_b15e6bd227209e964604d899704a2131/

https://www.addisonautoplex.com/
Slow (London, UK - EC2 agent) - https://webpagetest.org/result/210212_DiRK_e76a9e9e26cd35a3588175b529ef6463/
Normal (Dulles, VA agent) - https://webpagetest.org/result/210212_DiYE_0c50175c92b6141ce4b41f58b41b25d6/

(Scripts not used for actual tests, just to verify delay)

Is this a function of how much CPU is given to the VMs running the agents, or is another aspect causing this behavior?