[aristus]

You have good instincts here, but

1) In very large systems there are many many load balancers

2) Server "spikes" (eg swapping) can happen very suddenly, so distributing health info is a hard problem

3) Determining a priori which requests are heavy and light is also hard. Think about "SEO urls": they blow up the cardinality.

4) Even assuming perfect global knowledge, you are proposing to solve a variant of the knapsack problem.