Firstly - let me begin by saying that this is quite an amazing product and I can't wait to get to use it properly!
I'm waiting for my first printer to arrive and have installed OctoPi on a Pi 3B+. Watching the Gcode Viewer, it was painfully slow. I have a couple of sample prints from thingiverse that are about 20x30x10mm. Printing to the virtual printer, it started with an estimate of 5 mins, then after a few mins, changes to 5h50min. the gcode viewer shows a few mm of movement every few seconds. A few days later, tried again and it just finished in 5 mins, and the gcode viewer was at "full speed". another reboot, and it's back to slow again. I've gone through this cycle and haven't been able to figure this out.
I have rebuilt the entire pi twice more, once on a different pi, and had the same effect on all three builds.
One thing that seems suspicious is the occasional 0.5+ sec delay between the N-command and the "ok" in the serial log, as below:
2020-03-06 06:36:36,704 - Send: N1534 G0 F300 X93.3 Y95.098 Z4.628
2020-03-06 06:36:36,757 - Recv: ok
2020-03-06 06:36:36,764 - Send: N1535 G0 F9000 X93.1 Y95.601117
2020-03-06 06:36:36,830 - Recv: ok
2020-03-06 06:36:36,833 - Send: N1536 G0 X93.1 Y96.68621
2020-03-06 06:36:37,048 - Recv: Not SD printing
2020-03-06 06:36:37,971 - Recv: ok
2020-03-06 06:36:37,981 - Send: N1537 G0 X93.1 Y119.55644
2020-03-06 06:36:38,001 - Recv: T:200.00 /200.00 B:60.00 /60.00 @:64
2020-03-06 06:36:38,026 - Recv: Not SD printing
I've attached the logs, but I hope that this is just something stupid I forgot to do!