we are experiencing very slow WordPress admin panel, when we logged in & access the website, website taking too much time & sometime showing 502 Bad Gateway error.
I am having the same issue. I was on the phone with Hosting support for 2 hours yesterday and they ended up trying to blame the ISP for the issue. This of course makes no sense because if it were an ISP issue all websites that I try to go to would be slow or non responsive. Still no answer to this yet.
Last week, I moved all 4 of my sites (which had become unusable on GD's server for similar reasons and ZERO help from GD tech support), and my sites are working beautifully. Faster and better than ever. It is not us. It is not our sites. It is not our plugins. The only difference it that I took GD out of the equation.
That's what I'm going to do and get all my clients which I recommended using Godaddy (how embarrassing) to move with me. There is zero help on this hosting company when it comes to speed, they blame everything else but them selves. So your story for example is just one of many. Best solution is just to get something better I guess. It's just a migration pain I didn't want to get into.