4 and 5. Thank you. Although I think I found the problem as the other user altenbach suggested me to upload a simplified version of the VI for everyone to see and debug. As I was simplifying the VI, I removed the "Recording" value property node and the case structure in the monitoring loop. And it worked! I could plot all 8 waveform graphs without slowing down and causing problems with the monitoring loop. When I change the property note to local variable, the monitoring loop seems to be okay too and not being affected by the plotting. I wonder why though. The "Recording" boolean indicator doesn't seem to have anything to do with the waveform graphs. When I was using the value property node, plotting 1 waveform graph is okay. When plotting 2 or more it starts to slow the RT Monitoring loop down and eventually gives me the error.
Each property node forces LabVIEW to switch to UI thread and update the front panel. When writing to a terminal LabVIEW decides when it will update the frontpanel. The more property nodes you write the slower your program will get.