Six bottles of RUM: Real World Mobile Performance
summary
Measurements from real users show a cold TCP fetch for zero bytes taking 0-byte cold TCP ≈200–250ms, while a 6.7KB image drops to roughly 6.7KB image ≈100ms warm; average pages report 42 resources but many sites have 80–160, so serial costs add up. Cookies under keep cookies <400B have negligible cost but 4KB headers add ~50–60ms cold (≈20–25% warm). Domain sharding multiplies cold-connection costs—domain sharding hurts on mobile—HTTP/2 multiplexing and sprite usage (Amazon sprite tests) avoid those penalties; use cookie‑free domains and tune CDN/server timeouts.
AI-written summary · about
chapters (7)
- 0:00Introduction to RUM
- 4:58Why mobile performance matters
- 12:24HTTP performance basics
- 17:22Domain sharding pitfalls
- 21:11HTTP/2 and connection limits
- 23:41CSS performance myths
- 28:34Key mobile performance takeaways
