This is not so much a request for support, but may help others as I see many people on the forums have the same problem.
Problem: SiteGround hosting and getting 504 Gateway Timeout – Can’t Log In to WP Admin and also cannot load pages with lots of data. Simple pages still load. LOTS of errors show in the log.
PS: I am not a technical person so this is all layman’s experiences.
I have battled with this problem for months now, and progressively getting worse and many calls to SiteGround we achieved nothing including hiring a professional developer. All respect to SiteGround, I still recommend them, but maybe my site is just ‘different’.
The site gets very few visitors as its a closed/private training administration site, so maybe 5 people a day. It has 35 plugins all necessary, but handles lots of form submissions. Stores 60 000 plus submissions with attachments which are all offloaded to Digital Ocean for storage. Disk usage for the site is 15 GB, large I know. Pageviews less than 1000 a month. Bandwidth <45MB month.
EVENTUALLY I have resolved the issue and my site now loads like lightening again.
I removed the SG Optimiser plugin completely, Disabled NGINX DIRECT DELIVERY, DYNAMIC CACHE and MEMCACHED in my SiteGround account completely. Converted back from Ultrafast PHP back to Standard PHP. ALSO removed Jetpack completely.
HayHo, site works perfectly again. No errors show anymore in my logs.
If I activate ANY of the above again, I get 504 errors. Seems any of the speed enhancing tools actually slow things down.
Now according to the SiteGround engineers this is not possible and it cannot be happening, but as I said I am not a tech expert, jut know what I did to fix the problem.
This is my experience, if it helps anyone else, great.