UPDATE:

On another site with reasonable traffic I had this same issue. Traffic is not massive - 2.4k of unique visitors every month, so not a high traffic site.

I didn't realize that the monthly reports were failing due to exceeding memory limit. Even a Full Stats by Week crashed it which REALLY surprised me. We're talking 826 pages and 468 unique visitors. Memory limit WAS 128Mb. I am surprised that such a small report would take so many resources.

I had to increase memory limit to 200Mb to make the weekly report export to PDF. Even then it took 2-3 minutes to generate.

So this is an FYI to anyone suffering these issues. You need TONS in your PHP memory limit or it'll never work.