Google’s John Mueller said on Twitter that Web Light has no impact on your search rankings in Google. It is just a way for Google to display your content to those with slow devices, after they click from the Google search results to your web site. It has no impact on ranking in Google.

Web Light doesn’t change rankings, it’s purely a way of making your content more accessible to folks with lower-end devices & connections. You can check the impressions & clicks for it in Search Console now if you want to decide based on that 🙂
— John ☆.o(≧▽≦)o.☆ (@JohnMu) June 15, 2018
I get why some people think it is a page speed issue or maybe related to AMP and the carousel, but it is not. It is not new, we covered it ages ago and now it is for the first time visible as a filter in Google Search Console.
You have very little control if your pages are going to be served over web light or not. You can opt out of it, but outside of that, you cannot push people easily from Google search to your web light pages – nor would you want to. If you wanted to push them to something like it, you’d likely opt for AMP pages over web light.
So don’t worry about web light and Google rankings.
Forum discussion at Twitter.