Justuno vs OptinMonster: Shopify Performance Comparison
Justuno wins with Grade A vs Grade B — a one-grade 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 | Justuno | OptinMonster | Faster |
|---|---|---|---|
| Overall Grade | A | B | Justuno |
| JS Payload | 4.44 KB | 41.02 KB | Justuno |
| CPU Desktop | 3 ms | 9 ms | Justuno |
| CPU Mobile | 17 ms | 49 ms | Justuno |
| Layout Shift | 0.0 | 0.0 | Justuno |
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 →
OptinMonster — The Heavier Option (Grade B)
OptinMonster scores Grade B with a JS payload of 41.02 KB and 9 ms desktop CPU execution. Mobile CPU: 49 ms. Performance is acceptable but worth monitoring, particularly on stores with high mobile traffic share.
View full OptinMonster speed report →
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
When to Choose OptinMonster
- You need its specific features and can monitor its mobile impact
- Your other apps are lightweight (Grade A) so there is headroom
- You are on desktop-skewed traffic where CPU cost matters less
Performance Verdict
Winner: Justuno — Justuno wins with Grade A vs Grade B — a one-grade 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.