I worked for an analytics company at one point and we generated impression IDs server side (you'd be surprised how broken client-side JS randomness can be) and

I worked for an analytics company at one point and we generated impression IDs server side (you'd be surprised how broken client-side JS randomness can be) and included them in our loader. If we had been using CloudFlare I would have put our loader directly into this service worker so that we could shave a few more milliseconds off our intial response time, so that we could get to our cached JS faster. So yeah, if you care about milliseconds :)