It's definitely cheap to execute, the problem lies in the network overhead. With sub-resources (css, js, images) you can go from 1 request per page to 10 or 100 which is still negligible for fast connections (10 mbps+, http2) and servers with low request overhead - but the worst case scenario is high latency http1 connections where each request really matters.
However if you are serving clients with highly restricted bandwidth you're probably going to want extremely cacheable resources (public, immutable) and perhaps even a completely different site architecture.
However if you are serving clients with highly restricted bandwidth you're probably going to want extremely cacheable resources (public, immutable) and perhaps even a completely different site architecture.