Your experience reflects what many others have lived. In recent month there seems to have been a drop in Adsense performance driven by mobile.

Now the bad news...
You can't go back. The problem is that Google will serve your site to both desktop and mobile users. having a non-responsive site with non-responsive ad units will cause the ads to be shown on mobile devices that are against Adsense policies (eg: taking up most or all of the screen above the fold). This will lead to policy violations.

You could have a desktop site and then turn adsense off completely on mobile, but my guess is that you will make even less revenue than with responsive.
You could have two versions of your site, one desktop and one mobile. But again I don't see how this solve the problem.

The root cause of the problem is not the responsiveness of the site it is the fact that mobile users seem to click on fewer ads. Implementing page-level ads could help this situation.

Actually it *could* be the responsive, if your mobile version pushes that ads way too far down the page (which is what mine did - all the ads, which were in the sidebar, went below the content) Depending on how long a page you have, your mobile users might not even see them - check your Active View Vieweable in your AdSense Performance reports. Get some heatmap software (like crazy egg or hotjar - it's cheap) and see how many people are actually looking where your ads are placed. I ended up having my developer move the first sidebar ad into the content for mobile (and it wasn't trivial) and that certainly helped.

having a non-responsive site with non-responsive ad units will cause the ads to be shown on mobile devices that are against Adsense policies (eg: taking up most or all of the screen above the fold).

My sites, for the most part, are not responsive. The pages that are responsive make much less than the others. I worried about the non-responsive ads taking up too much space at the top of the page on mobile devices but was told that because the sites are not designed as responsive, then the "too much space" rule doesn't apply. It would only apply if the site were designed to be responsive.

That rule may change at some point. Who knows. And at some point Google may say that only responsive sites can implement their ads. Again who knows. But for my readers, most of whom are Boomers with desktops, the non-responsive status quo works.

@ember here is an example of someone who did receive a violation for not having a responsive or mobile optimized site. [webmasterworld.com...]

That post was actually why I contacted Adsense about the issue. I can only go by what I was told when I specifically asked the question and had an Adsense person look at my site. If they come after me, I'll deal with it then.

having a non-responsive site with non-responsive ad units will cause the ads to be shown on mobile devices that are against Adsense policies (eg: taking up most or all of the screen above the fold).

Non-responsive ad units on non-responsive sites are very small because these sites don't have a meta viewport tag.

@ember here is an example of someone who did receive a violation for not having a responsive or mobile optimized site. [webmasterworld.com...]

No no, that's not what the e-mail said. Not having a mobile optimized site is not in itself a violation. You are, however, responsible for how the ads appear on your users' various devices.

My sites, for the most part, are not responsive. The pages that are responsive make much less than the others.

IMO you will not be able to get a realistic account of how responsive pages perform unless your entire site is responsive. This is because the mobile audience will likely leave once they navigate to a non-responsive page, never getting to other responsive pages. You want the user to stay on your site and load lots of pages with lots of ads :)

did all of you see and increase in Adsense performance since switching to responsive? I don't know if it is me only, but I have a feeling this responsive concept is not as cool as everyone sells it for. At least not for purely informational sites.

I have an info site. I saw an almost immediate increase in Adsense performance after switching to responsive, but I made the entire site responsive.

Also, remember being "responsive" allows the presentation to adjust properly to various screen sizes. This basically means mobile phones since most tablets (unless very small) display the desktop resolution.

A large part of mobile phones is social media and apps, so that's a target when responsive. If you don't exploit that audience, you'll not fully appreciate that target potential. So go out and get that audience.

Open social media accounts and get involved with those communities. Test drive the dozens of apps and learn how your content may be included in those metrics. Make your site valuable to them. And watch your server logs to make sure you are not blocking these traffic sources.

did all of you see and increase in Adsense performance since switching to responsive? I don't know if it is me only, but I have a feeling this responsive concept is not as cool as everyone sells it for. At least not for purely informational sites.

I also run an informational site. I have not seen an increase in adsense performance, but I have seen a huge increase in mobile traffic. Since fully upgrading my site to fully responsive using a Bootstrap framework, my traffic has grown 3 fold and my device mix is now 60% mobile 40% desktop. Up to this month, most of my revenues have been from desktop. I am starting to see a shift where I am now getting 50% of my revenue from mobile.

I agree with what keyplyr says, if you do not go fully responsive you will miss out on a large segment of your audience. Your piece of the pie will continue to shrink over time.

I switched back all the ads below the fold to non-responsive ad units & left the ads above the fold responsive. I hope I'll see improvement in earnings

for my readers, most of whom are Boomers with desktops, the non-responsive status quo works.

That's been true for us, too. Our information site reaches people who are at various stages in the research/planning/doing cycle, and the overwhelming majority of our revenue is earned or booked during the research/planning phases. (The "doing" phase doesn't seem to be conducive to ad or affiliate revenue, although that could change in the future.)

Our main site isn't responsive: Instead, about a thousand of our most popular and useful pages have hand-optimized mobile versions. We've run AdSense ads on the mobile pages from time to time, but the revenue has never been enough to justify the screen space. At this point, I've come to regard our mobile pages as loss leaders and as a service to readers who want mobile-friendly content during the "doing" phase.

YMMV.