How much is your title weight? Other than the title, the nail polishes are as good a result for “lampe” than the lamp is, and there’s little reason for Relevanssi to prefer it. Try increasing the title weight a lot – add a zero or two at the end of the weight. Does that help?
(By the way, I’m pretty sure a different kind of search results template would improve the user experience a lot. Most of your products are very similar and only the colour is different. Being able to get an overview of 100 different colours on one page would be great, and a much better user experience than browsing through tens of pages. As an easy first step, I’d at least increase the number of results shown on one page from 15 to at least 50.)
Do yo have the throttle enabled in the Relevanssi searching settings? If it is, try disabling it. Does that help?
