This flicker can be managed in multiple ways:

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The easiest solution is to use a top-notch personalization tool that optimizes the load time of each embedded content piece, resulting in load times of under a second—this way, most visitors won’t notice the flicker.
The second option is simply being careful about the size of the content being loaded above the fold. Changing a headline, for example, is quick and lightweight, but embedding a big, colorful hero banner can take significantly more time.
You can also hide the native, base content until the embedded content has time to load. This solution is less popular, since visitors are used to load times of under a second. Many marketers prefer to load the base content as quickly as possible to reduce the chances of visitors bouncing.
Some choose to only use embedded personalized content below the fold, where a delay of a second or two doesn’t matter, since users won’t typically scroll down that quickly. While this is a perfectly valid solution, it means less personalization in your best virtual “real estate.”
The fifth option is to use server-side personalization, which means generating the optimal, personalized content before the HTML is sent to the visitor’s browser. While this provides the best user experience in theory, it’s the least popular solution due to its technical difficulties. It usually requires a complete IT switzerland customer email list team and a lot of backend development, meaning it’s only used by the top 0.1% of online businesses.
The good news is that the first two options—using a top-notch personalization tool like OptiMonk and being careful about the size of the personalized content—will usually result in flickers that are basically unnoticeable, providing a nearly perfect experience from the customer perspective.

Another popular way of managing speed issues is using overlays to deliver personalized messages whenever possible.

Popups and other overlays don’t have to appear at the same time as the native, base content. In fact, delaying overlays by a couple of seconds actually makes them more efficient and results in better conversion rates.

But of course, the use of overlays needs to be limited, otherwise, the visitor will feel flooded with “annoying popups.”