Ultimate Sales Boost — Performance Grade D
Critical — avoid on mobile stores
Ultimate Sales Boost is a post-purchase upsell and cross-sell app for Shopify that scores Grade D overall in StackConflict’s independent performance lab. The grade is driven entirely by its download payload: our lab recorded 645.46 KB of JavaScript — an oversized JavaScript bundle that exceeds the entire third-party JS budget recommended by Google, placing it in the D-tier for network cost. Notably, once downloaded, the script executes very efficiently: only 55 ms of desktop CPU time (Grade B), meaning the bottleneck is connection speed and first-paint delay, not main-thread execution.
The practical impact for Shopify merchants is a longer first-paint on slower connections. A 645.46 KB download must complete before the script can initialise — on a mid-range mobile connection (10 Mbit/s) this adds roughly 504 ms of network latency before the browser even begins execution. Once the script does load, it runs quickly: StackConflict’s 4× mobile throttle measurement recorded only 318 ms (0.32s) of CPU execution — well within acceptable limits. For merchants on high-speed connections or desktop-dominant traffic, the payload cost is far less noticeable than on mobile-heavy, emerging-market stores.
Verdict (Grade D — Critical): If Shopify store speed and mobile conversion are priorities, merchants should evaluate lighter alternatives before committing to Ultimate Sales Boost. StackConflict has benchmarked Zipify OCO and Aftersell in the same upsell category — both carry Grade A or B ratings with significantly lower payload and CPU profiles. The performance difference compounds on mobile-first stores where JavaScript parse, compile, and execution times are amplified by device limitations.
Lab Results
| Metric | Value | Grade |
|---|---|---|
| JS Payload | 645.46 KB | D |
| CPU Execution (desktop) | 55 ms | B |
| CPU Execution (mobile) | 318 ms | — |
| Cumulative Layout Shift | 0.0006 | A |
| Overall Grade | — | D |
| Category | Upsell | — |
| Last tested | 2026-05-30 | — |
Lab Verdict
Grade D driven by large download payload. High conversion risk 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 |
Faster Alternatives
These upsell apps have earned a Grade A or B in our lab and are drop-in alternatives worth evaluating:
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.