Conversion lag refers to the delay between people clicking an ad and when they purchase a product on the website. This lag can be significant and changes between niches & websites. This lag creates a late conversion which is attributed to the original ad click.

Conversion lag happens because the user's purchase journey is not linear most of the time. Most users purchase journey doesn't look like this:

It looks much more complicated, something like this:

Ad click -> didn't purchase -> 2 days later came from organic search -> didn't purchase -> saw an ad -> came from direct -> added to cart and didn't purchase -> 3 days later saw retargeting ads on high frequency -> came from organic -> purchased.