Track performance issues with CDNs

[jdno]

Once in a while, we get reports from users that they are experiencing performance issues with our CDNs. We currently neither have good visibility into the frequency of these issues nor do we have good tools to gather data. We will use this issue to collect reports and look for patterns.

Reports

Zulip thread	CDN	Reproducible
search incredibly slow for nightly-rustc docs	CloudFront	No
rustup is slow