MailerLite — Performance Grade B
Acceptable — monitor on mobile
MailerLite is a email marketing and automation app for Shopify that scores Grade B in StackConflict’s independent performance lab — placing it among the better-performing apps in the Email Marketing category. With a moderate JavaScript bundle that is acceptable but worth monitoring as your stack grows (48.68 KB compressed) and negligible main-thread time — it will not compete with your theme or checkout scripts for CPU (9 ms desktop), it adds minimal friction to the storefront experience.
On mobile devices StackConflict’s 4× throttle pass measured just 15 ms (0.01s) — a direct measurement on a simulated mid-range Android device. This negligible overhead means MailerLite will not meaningfully affect your store’s Time-to-Interactive or Core Web Vitals scores. Merchants running multiple apps can safely stack MailerLite without it becoming the bottleneck in their combined performance budget.
Verdict (Grade B — Acceptable): MailerLite by MailerLite is a safe choice from a Shopify site speed perspective. It passes all StackConflict thresholds for payload, CPU, and layout stability. Install it with confidence on any storefront, including high-traffic mobile-first stores where JavaScript overhead directly influences Add-to-Cart conversion rates and Google’s Core Web Vitals ranking signals.
Lab Results
| Metric | Value | Grade |
|---|---|---|
| JS Payload | 48.68 KB | B |
| CPU Execution (desktop) | 9 ms | A |
| CPU Execution (mobile) | 15 ms | — |
| Cumulative Layout Shift | 0.0006 | A |
| Overall Grade | — | B |
| Category | Email Marketing | — |
| Last tested | 2026-05-30 | — |
Lab Verdict
Download size is within acceptable range. Monitor conversion impact on mobile storefronts.
Grading Scale
Each app is graded across three independent dimensions. The final grade is the worst of the three — no dimension can be hidden by a good score elsewhere.
| Grade | Payload | CPU (desktop) | CLS | Risk |
|---|---|---|---|---|
| A | ≤ 30 KB | ≤ 50 ms | < 0.1 | Lightweight — zero friction |
| B | ≤ 150 KB | ≤ 150 ms | < 0.25 | Acceptable — monitor on mobile |
| C | ≤ 500 KB | ≤ 500 ms | < 0.5 | Heavy — high conversion risk |
| D | > 500 KB | > 500 ms | ≥ 0.5 | Critical — avoid on mobile stores |
About This Test
StackConflict measures app performance in a clean-room environment. Each app’s JavaScript bundle is injected into an owned Shopify development store via a Playwright-controlled browser, with no other third-party apps installed. V8 CPU cost is the ScriptDuration delta measured before and after injection on the same live page. Mobile CPU estimate applies the 4× Lighthouse slowdown factor to approximate real-device impact.
Data sourced from the StackConflict Lab. Last updated 2026-05-30.