Does Optimizely slow down a store?
In 7IT’s test of 367 US online stores, Optimizely ran on 21 of them (5.7%). On the median store that loads it, it downloaded 99 KB and kept the phone busy for 0.10 s before the page could respond.
5.7%of 367 US stores run it
99 KBmedian download on stores that load it
0.10 smedian time the phone spends running it
Stores with it and without it.
| Stores | Median score | Main content | Blocking time | Apps on the page | |
|---|---|---|---|---|---|
| With Optimizely | 21 | 30 | 11 s | 2245 ms | 28 |
| Without | 346 | 35 | 11.9 s | 1222 ms | 20 |
Stores that run Optimizely had a median score of 30, against 35 for stores without it. They also run more apps overall (a median of 28 against 20), so that gap is not all Optimizely’s doing. The size and phone time above are Optimizely’s own. On this sample there is no score penalty that can be pinned on it.
One in four stores that load it download more than 105 KB of it, and spend more than 0.30 s running it. Median Google phone-lab results, home pages only, September 2026. Method and limits in the study.
If you run Optimizely.
- A/B testing scripts often hide the page until they decide what to show. Run tests server-side or at the edge where possible, and keep the anti-flicker timeout short.