Connection lost, taking a long time to submit data.

Hi,
I’m using wordpress without any problem until I update to 4.7.4 and also update Avada to 5.1.6. The problem is, every time when I try to publish or update a page, it will take about 3-5 minutes after I click publish or update, the page just stuck, and my whole site is stuck, I can’t open any page or do any thing. But when I checked Godaddy server at the same time, everything is fine, nothing overload. After 3-5 mins, the page will successfully published or updated, but this just really annoying, I have a lot of event page need to update at a short period. Sometimes the page header will even show a “Connection lost, Saving has been disabled until you’re reconnected. We’re backing up this post in your browser, ” warning.
Same problem happened when I submit a large Ninja form. For a small Ninja form with 10 fields, it will just take a second as usual to submit. But when I submit a large form with 30-40 fields, the page just stuck like when I update a page, the form will be successfully submitted, but no matter what kind of action I set (like redirect or send an email notification) it won’t work, it just refresh the page. At before, the form was working great. Can anyone help me with this? I’m not sure what’s wrong. I have been disable all the plug-in, it still happen. Only if I switch a theme, the page will update and publish in a second, but the Ninja form problem is still the same.
Looks like it is when I try to submit a large amount of data at the same time, wordpress just crash or lose connection, or overload. But I can’t find where is the problem.