This article provides brief information when the Signal wait time percentage is in warning or critical state and there is no CPU pressure (via CPU utilization).
Signal waits are an indication of possible internal CPU pressure. The CPU Signal Waits percent is a ratio metric comparing signal waits to total waits as a percent. That means you can see a spike in signal waits from one minute to the next without the server itself showing a high CPU utilization for the server. Since this is a percentage, a reduction of overall waits within an instance can cause this ratio to increase if signal waits remain fairly constant. This is more likely to show up as warning or critical in very lightly used systems.
With a virtual database server this could also mean that your guest O/S is waiting for an available vCPU to be assigned, so you should check the VM Ready Time (VMware) metric as well to see if that is the case.
Another metric to look at within vSphere would be the co-stop time, this is the amount of time the host is taking to schedule vCPUs to guests. We don't have this metric in DPA yet, so you will need to go to vSphere to see this (VMWare).
Recommendation: If your system is not under CPU utilization pressure, you can adjust the default thresholds for this metric to eliminate the false alarms and noise. Resources > CPU > Signal Waits Percent (settings).