An AI builder ships a website in an evening. The catch shows up in speed and SEO. A real example from our studio: a client's site loaded its main content in 18.7 seconds on mobile, Performance 54/100. After a rebuild on custom code we hit 1.5 seconds and 99/100. Below are the exact numbers, what caused the slowness, and when a builder is enough.
I'm Tair Khamitov. I run DevNova, a web studio in Bratislava, and roughly 15 audits of other people's sites pass through my hands every month. This one was among the clearest I have seen all year, so I'll walk through the whole thing. The numbers, and the part where I got it wrong at first.
The hype: a website in minutes, just tell the AI
2026 sounds simple: tell an AI builder what you want and a few minutes later you have a finished site. And visually, it can look decent. The problem is that what you see on screen is not what a visitor feels on a phone over 4G. Between 'looks done' and 'runs fast' sits a gap a builder rarely closes for you.
The reality: what we measured
A B2B company found us all the way from Kazakhstan, through our profiles on Clutch and TechBehemoths. Their site was built in the AI builder Mobirise. I'll be honest: on desktop it looked fine, I would not have touched it. Then I opened it on a phone over normal 4G and waited. And waited. The main content showed up after almost 19 seconds. So we measured it in Google PageSpeed Insights, before the rebuild and after:
| Metric (mobile) | Original site (AI builder) | After rebuild (DevNova) |
|---|---|---|
| Performance | 54 / 100 | 99 / 100 |
| LCP | 18.7 s | 1.5 s |
| FCP | 5.3 s | 0.9 s |
| CLS | 0 | 0 |
| Accessibility | 77 | 93 |
| SEO | 91 | 100 |
Source: Google PageSpeed Insights, mobile, before/after. First-party measurement (DevNova).
In plain terms: LCP is how long the main content takes to show (Google wants it under 2.5 s). FCP is the first thing that appears on screen. CLS 0 means nothing jumps around while the page loads, which was already fine here. The number that mattered was the drop from 18.7 to 1.5 seconds.
Why was it so slow when it looked good? BLUNTLY: not on purpose. It came from layers nobody switched off. The builder loaded the whole of Bootstrap, animation libraries and a set of plugins, a 2GIS map widget (over half a megabyte) and full-size uncompressed images. The images alone could be cut by more than 2 MB. On top of that, no caching. The page weighed 3.5 MB. That is what a page with video should weigh, and there was no video here.
An AI builder doesn't build a slow site on purpose. It builds it from layers nobody switches off for you.
The brief was a hybrid landing page with a branch for each service. We rebuilt it from scratch on custom code (web development). We stripped the bloat. Compressed the images. Cached everything, deferred the scripts. The result is in the table above. Price: Landing, one-time €349. Because it was an unusually good collaboration, we added extended keyword indexing and automatic forwarding of form enquiries straight to their company inbox as a bonus. The full case with before/after photos is on the project page.
The fix: how to tell if your site is the problem
If you were sitting across from me, here is the first thing I would do. It takes ten minutes and costs nothing:
- Measure reality. Open PageSpeed Insights on mobile. If LCP is over 2.5 s, Google already treats it as poor Core Web Vitals and it is holding you back.
- Look at what loads. If you see Bootstrap, an animation set, heavy widgets and full-size images, that is your builder tax.
- Decide soberly. Small tweaks will not survive a builder; real numbers change only with a rebuild on custom code. See the pricing or the DevNova vs Wix comparison.
FAQ
When is an AI builder genuinely enough? For a personal portfolio, a one-off event or a page with no traffic or SEO demands, it is fast and cheap. For a business meant to bring in enquiries, its limits show.
Why did the site look good but run slow? Design and speed are two different things. A builder optimises the look, not what is under the hood.
Why is a Mobirise site slow? Mobirise assembles the page from ready-made blocks and ships the whole of Bootstrap, animation libraries and plugins with them, including the ones you never use. Add full-size images with no compression. Nobody switches that off for you, so it all loads at once. That is exactly what we saw here: 3.5 MB and an 18.7s LCP.
How much does a rebuild cost? Landing from €349, one-time. Exact quote in the brief. See live regional pricing in the DevNova Index.
Will I lose content or SEO? No. We carry the content over, and SEO usually improves (here the score rose from 91 to 100).
How long does it take? Our standard is 11 days.
Author: Tair Khamitov, founder of DevNova, a web studio in Bratislava that builds sites and automations for small and medium businesses.
Got a builder site and a hunch it is slow? Send a brief and we will measure it and tell you straight whether a tweak is enough or a rebuild pays off.