Wow, no idea… that’s a tricky one to troubleshoot. Last 2 suggestions that come to mind: check server error logs to see if PHP is reporting anything, and troubleshoot plugins by disabling them one at a time then testing load speed. Of course, the problem could also be due to plugins interfering with each other, so it might be worth disabling ALL the plugins and testing load speeds you turn each plugin back on.
What a pain…
