That’s the problematic part. The traffic flows directly between customers and nodes without any middleman (except when they using a Gateway MT). So you actually cannot check the speed. It also depends on a distance (and number of hops) between the node and the customer. The speed between your node and the customer from the next apartment likely will be higher than between your node and the customer on the other point of Earth.
The speed between the node and auditors doesn’t matter, unless we implement a distributed network of independent auditors (see Distribute audits across storagenodes). The distributed network of auditors could be used to get some believable statistics, which could be difficult to trick. However it’s difficult to implement.