Lets assume that ISP's don't throttle/restrict traffic in any way other than physical limits.
Scenario one: path from home to website
Home -> ISP -> heavily utilised link -> ISP -> web site
Scenario two: path from home to website via VPN
Home -> ISP -> underutilised link -> VPN provider -> underutilised link -> ISP -> website
Wouldn't your test identify throttling in this case even if none existed? Or does over utilisation of links/equipment count as throttling? This is a simple example - in practice there are many more links and potential points for bottlenecks.