CDN Latency Optimization: Reducing Time to First Byte

May 22, 2026 · James Thornton · 4 min read

Understanding CDN Latency Components

CDN latency is the sum of several components. To optimize it, you must understand and measure each one: For foundational context, read CDN Performance Metrics: What to Measure — FlyFish NWA.

Total TTFB = DNS + TCP + TLS + Edge Processing + (Origin Fetch on miss)

Cache HIT:  5-50ms = DNS(0-20ms) + TCP(5-15ms) + TLS(5-15ms) + Edge(1-5ms)
Cache MISS: 50-500ms = above + Origin(50-500ms)

DNS Optimization

DNS resolution adds 10-50ms for cold lookups:

Performance Comparison: Edge vs Origin Processing
Metric	Edge Processing	Origin Processing	Improvement
Latency (p50)	8–15 ms	120–300 ms	15–20x faster
Cache Hit Ratio	85–95%	N/A	Reduces origin load
TLS Handshake	1–5 ms (nearby PoP)	50–150 ms	10–30x faster
Bandwidth Cost	$0.01–0.04/GB	$0.05–0.12/GB	3–5x cheaper
Availability	99.99% (multi-PoP)	99.9% (single DC)	10x fewer outages

Figure 1: CDN Performance Dashboard — Core Web Vitals with CDN optimization

Lower DNS TTL cautiously — Balance between failover speed and cache hit rate. See DNS TTL guide.
Use DNS prefetching — <link rel="dns-prefetch" href="https://cdn.example.com">
Minimize DNS lookups — Serve all assets from one CDN hostname instead of multiple

Connection Optimization

TCP + TLS handshake adds 20-100ms:

TLS 1.3 — 1-RTT handshake instead of 2-RTT for TLS 1.2. Configure minimum TLS 1.2.
0-RTT resumption — TLS 1.3 supports 0-RTT for repeat connections. First request after session resumption has zero handshake latency.
HTTP/3 (QUIC) — Combined transport + crypto handshake in 1 RTT, or 0 RTT for repeat connections. See HTTP/3 guide.
Connection coalescing — Reuse existing connections for multiple domains. See connection coalescing.

Edge Processing Optimization

Processing at the edge typically adds 1-5ms:

Figure 2: Request Timing — Latency breakdown across the CDN stack

Simplify WAF rules — Fewer, more targeted rules process faster. See WAF configuration.
Optimize cache key — Simpler cache keys mean faster lookups
Minimize edge transforms — Each transform (compression, image resize) adds processing time

Cache Hit Ratio Impact

The single biggest latency factor is whether the response is a cache hit or miss. Improving cache hit ratio from 80% to 95% means 75% fewer users experience cache-miss latency.

Origin Latency Reduction

For cache misses, origin response time dominates:

Use an origin shield — Shield location close to origin reduces edge-to-origin RTT
Keep-alive connections — Persistent connections between CDN and origin eliminate per-request handshake overhead
Origin compute optimization — Faster origin responses directly reduce miss latency
Edge caching of dynamic content — Even 1-second micro-caching eliminates most origin requests

Network Path Optimization

CDNs optimize network paths:

Anycast routing — Anycast finds the topologically closest PoP
Private backbone — CDN-owned backbone between PoPs avoids congested public internet paths
Smart routing — Real-time traffic steering to avoid network congestion (Cloudflare Argo, Akamai SureRoute)

Preloading and Prefetching

Reduce perceived latency by fetching resources before they're needed:

<link rel="preconnect"> — Start TCP+TLS handshake early
<link rel="preload"> — Fetch critical resources immediately
103 Early Hints — CDN sends preload hints before the main response

See prefetch and preload strategies for implementation details.