I agree with clembo.
Here's another few things to check:
1. AMP version
2. VisualRF setup -> what's the current memory allocation for VisualRF?
3. VisualRF logs -> if you tail /var/log/visualrf/visualrf.log, you should see a constant feed of information being updated for calculation
4. Types of devices
5. Number of floor plans
6. Number of devices placed on floor plans
7. AMP hardware resources (disk space, disk speed, RAM)