joined:Apr 20, 2017
I am sorry about those having lost traffic after switching to https. Personally I had no negative impact when I switched 1 year 1/2 ago. I didn't lost any traffic. My traffic is slightly up, but I don't think it has anything to do with the switch itself.
Now, for all those who report lost of traffic, maybe there are other factors. Like for example, it looks like Google did lot of "experimentations" since some months, and maybe your sites were impacted negatively by this. It's also possible that Google is giving more weight to response time of sites. Https is adding more latency, it can be neglected when your site run on a dedicated server and http/2, but if you are using shared hosting, may be this can be significant, if the server has a high load.
If you site has plenty of external files, (images, css, js) ,this can also increase the loading time, which might penalize your google ranking (honestly, I have no idea of the real impact, I just know it's one criteria among tons of others). For example, I know that the recommendation is to have CSS and JS in external files, to be cached, but after intensively experimentation I found out that (in my case), it's better if I include the CSS and JS into the page header. Using http/2 and gzip compression, it's adding only very few extra bytes and avoiding plenty of blocking network connections. I also lazy load all images.
Also, be sure that your SSL is configured correctly and in an optimal way. I test my site with [ssllabs.com
...] (there are certainly other sites like that). It analyses your SSL connection and certs and reports if there are errors or things like that. Also, there are optimizations which can be done at the level of the web server (software), like removing some cypher algorithms which are not enough secure, or not longer used.