Large question banks have always caused some issues but Moodle has been working on the back up and restore and the new backup format has helped things considerably (this has been an experimental setting but I recommend trying it).
Unfortunately, new minimum requirements is a global thing, not just limited to Moodle. I disagree that Moodle is less efficient with each release - there has been a lot of work done to make it more efficient. But, like any website, if your host is limiting resources, then it is going to respond negatively.
As for no error message, do you have Developer Debugging turned on - that should give you a very detailed error message and help you troubleshoot the message. It is true that sometimes questions get corrupted and it is often just one or two questions that are causing all the issues.