Why slow 4G is the honest test
Your campaign is usually approved on office Wi-Fi and a large screen. A meaningful share of your audience opens it in conditions nothing like that: indoors, in transit, in rural coverage, on unstable 4G. Simulating a slow connection is not pessimism — it is the realistic floor of the experience you ship.
The cost of slowness is concrete. If the hero image or the offer takes ten seconds to appear, many recipients have moved on before your key content ever rendered.
An email that performs on slow 4G performs everywhere. The reverse is not true.
The two metrics that matter
The first visible element: logo, opening line. It tells the reader the email is coming.
The moment the main element — hero, product visual, large headline — is visible.
Both metrics are simulated on a mid-range smartphone over a slow 4G connection, as in Google Lighthouse mobile audits. It is the realistic floor of the experience you ship, not the best case.
How to read a loading speed check
5 best practices
- Optimize images: 700 px wide at most, under 150 KB, JPG by default.
- Compress and resize visuals to their display size — never let the email client scale down a 2,000 px file.
- Place a lightweight logo or a short text line above the hero: a fast first signal (FCP) reassures while the rest loads.
- Remove redirects from image URLs, host assets on a fast CDN and limit external fonts.
- Keep key elements — headline, CTA — as HTML text, so the message exists even while images are still loading.
Common mistakes
- Approving the campaign on office Wi-Fi only, and never on a phone over a weak connection.
- Building the hero as one large image containing the headline and the CTA: nothing readable appears until it loads.
- Exporting PNGs for photos, which are often several times heavier than a well-compressed JPG.
- Chaining image URLs through tracking or CDN redirects before the file is served.
- Relying on custom web fonts without a solid fallback font.
How Inbox Maestro checks it
Inbox Maestro measures the FCP and LCP of your test email on a simulated slow mobile connection, identifies the exact element slowing the render, and details its size, format and origin — so you know which file to fix first. The result appears in the Checkup section of your report, under Composition.
→ Compress / resize the image (≤ 700 px, < 150 KB), avoid heavy GIFs.
Frequently asked questions
How fast should an email load on mobile?
The main content (LCP) should appear within 4 seconds on a slow connection. Between 4 and 10 seconds the experience degrades; beyond 10 seconds, many recipients will have moved on.
What are FCP and LCP in email?
FCP is the moment the first element becomes visible; LCP is the moment the main element, usually the hero image, renders. Together they describe how the loading experience feels.
What slows email loading the most?
Oversized and under-compressed images, image URLs behind redirects, slow hosting and heavy animated GIFs. Image optimization is almost always the biggest lever.
Why test on a slow connection rather than Wi-Fi?
Because a meaningful share of opens happens with limited connectivity. An email that performs on slow 4G performs everywhere; the reverse is not true.