Caption vs Justuno: Shopify Performance Comparison
Justuno wins with Grade A vs Grade D — a significant performance difference that compounds on mobile devices.
Both apps provide popup, email capture, and conversion optimization functionality for Shopify stores. This page presents StackConflict’s independent lab results for both — measured in a clean-room environment with no other apps installed.
Head-to-Head Metrics
| Metric | Caption | Justuno | Faster |
|---|---|---|---|
| Overall Grade | D | A | Justuno |
| JS Payload | 517.67 KB | 4.44 KB | Justuno |
| CPU Desktop | 76 ms | 3 ms | Justuno |
| CPU Mobile | 127 ms | 17 ms | Justuno |
| Layout Shift | 0.0 | 0.0 | Caption |
Mobile CPU measured with 4× CDP throttle (mid-range Android). Tested 2026-05-30.
Justuno — The Faster Option (Grade A)
Justuno scores Grade A with a JS payload of 4.44 KB and 3 ms desktop CPU execution. Mobile CPU: 17 ms. It passes every StackConflict threshold for payload, CPU, and layout stability — making it safe to install on any storefront including mobile-first stores where Time-to-Interactive directly impacts conversion.
View full Justuno speed report →
Caption — The Heavier Option (Grade D)
Caption scores Grade D with a JS payload of 517.67 KB and 76 ms desktop CPU execution. Mobile CPU: 127 ms. This places it in the heavier half of StackConflict’s benchmark dataset — merchants running mobile-first stores should evaluate whether the feature value justifies the performance tradeoff.
View full Caption speed report →
When to Choose Caption
- Its unique feature set has no lighter equivalent in StackConflict’s database
- Your store traffic is predominantly desktop
- You have already removed or lightened other heavy scripts to compensate
When to Choose Justuno
- You are running a mobile-heavy store and Core Web Vitals matter
- You want to stack multiple apps without worrying about combined JS cost
- Conversion rate optimisation is a key priority
Performance Verdict
Winner: Justuno — Justuno wins with Grade A vs Grade D — a significant performance difference that compounds on mobile devices.
About This Test
StackConflict measures each Shopify app in an isolated clean-room environment — a single development store, no other scripts, Playwright-controlled browser. V8 CPU cost is the ScriptDuration delta before and after injection. CLS is captured from the same browser session. Results represent the median of three independent runs with outliers removed.
Data from the StackConflict Lab. Last updated 2026-05-30.