As a part of answering a question in another thread, I just now ran a simple Custom Report in both RM7 and RM8. My RM8 database is an import from RM7, newly imported nearly every day. The report ran in less that a second in RM7 - too fast to time. The exact same report ran a little over a minute in RM8.
I have a custom Obituary fact in my database to store and report on obituaries. My custom report for this test had four fields - 1) the person’s name, 2) the date from the obituary fact, 3) the description field from the obituary fact, and 4) the note from the obituary fact. I have a group defined with Obituary => Exists => Is True which is a useful filter for People List View and for Custom Reports and is the list of people I used in the report in both RM7 and RM8.
I did try one experiment in RM8 to speed things up. Namely, I removed the Obituary Note field from the report, even though the Obituary Note was whole point of running the report. In this case, the time was still noticeably longer than with RM7 - like maybe a second and a half to two seconds instead of sub-second - but the time to run the report was completely acceptable. It was much faster than it was when the Obituary Note field was included.
I had a similar performance problem recently with RM8 where I just did a search on Birth => Exists => Is False. When I looked at that performance problem with Windows Task Manager, there were a couple of seconds where RM8 was using 100% of a processor core CPU time, then there was about 30 seconds where it was reading the RM8 database, and finally about 60 seconds where it was using 100% of a processor core CPU time again. In the case of the custom report for Obituary Notes that was slow slow, it was using 100% of a processor core CPU time the whole time and there was no evidence that disk access was a bottleneck.