Magento Performance: Magento + Varnish Diagnostics

Running Varnish doesn’t automatically mean your Magento store is fast.

In many audits, Varnish is enableda but not effective.

Here’s a practical diagnostic + fix guide you can use in real projects

Step 1: Confirm Varnish Is Actually Working

Check response headers:

• X-Magento-Cache-Debug: HIT/MISS
• Age header present and increasing

If most pages = MISS → Varnish is acting like a proxy, not a cache.

Fix:

• Verify Magento cache type = Varnish
• Confirm correct VCL version deployed

Ensure traffic is hitting Varnish port (not bypassing to backend)

Step 2: Check Cache Hit Ratio (VIMP)

Use varnishstat

Look at:

• cache_hit
• cache_miss

Healthy pattern:

• Catalog traffic should have a strong hit ratio (often 75%+)

Red flag:
High traffic + low hits = fragmentation or over-purging.

Fix:

• Reduce unnecessary cache flushes
• Audit cookies and query params

Step 3: Detect Cookie-Based Cache Bypass

Common issue: cookies silently kill caching.

Symptoms:

• HIT in curl, MISS in browser
• Logged-out users missing cache

Quick test:

curl -I URL

curl -I -H “Cookie:” URL

If hit ratio improves without cookies → fragmentation confirmed.

Fix:

• Strip non-essential cookies in VCL (these can result in multiple versions of a page)
• Whitelist only required Magento cookies
• Audit analytics/personalization scripts

Step 4: Identify Multiple Cache Variants

Same URL cached multiple times due to:

• Cookies
• Currency
• Store view
• Tracking query params
• Device headers

Symptoms:

• Low hit ratio despite traffic
• High object count in Varnish
• Random performance per user

Fix:

• Avoid device-based hashing unless required
• Standardize currency strategy

Step 5: Check Invalidation Strategy

Over-purging is a silent performance killer.

Symptoms:

• Cache constantly cold
• CPU spikes during product updates

Fix:

• Audit modules triggering clean cache calls
• Avoid admin habits of manual flushes

Step 6: Measure Real TTFB

Cached pages should load extremely fast.

If cached TTFB is still slow:

• Backend leaks exist
• Network/CDN issues
• Compression or TLS misconfig

Check Google Search Console to get average response time (0.3 seconds).

Fix:

• Test origin (PHP code execution) vs edge TTFB (page from Varnish)

Advanced Diagnostics (When Needed)

• varnishlog → Inspect hash inputs
• varnishstat → Memory + hit trends
• Temporary debug headers in VCL
• Backend request monitoring during load

High-Impact Fixes (Real-World Wins)

In most Magento audits, biggest gains come from:

• Cookie normalization
• Query param cleanup
•  Smarter invalidation
•  Removing unnecessary cache variations

These alone can:

• 2–5x hit ratio
• Reduce backend load massively
• Stabilize TTFB under traffic