Hello David
I think we’re addressing two different issues here. The first is, that there are cases where you have a 1Gbps link and it seems underutilized at only 200 Mbps for example. Now for this issue, I had an extensive conversation with @ReneMolenaar and it turns out that there is a major difference in how routers and switches handle queueing. What I described in my previous post is correct for the most part for routers. However, the behavior of switches is much more complicated. We now have several NetworkLessons notes that clarify these issues:
Now the other issue has to do with how a port perceives congestion. Concerning your statement:
This is true assuming that the arrival of 100 packets on a port occurs at a rate that will trigger a threshold (whether in a simple manner on a router, or in a more complex manner on a switch). What triggers that threshold? The rate at which the packets arrive. If 100 packets each a size of 1500 bytes arrive within 1 ms on a port, that’s 1.2Mb in 1 ms (check my math please 
). Converting to seconds, that comes to 1.2 Gb/s on a GigabitEthernet port, which means you have congestion, which means you have queuing which means the voice packet will be prioritized.
If the 100 packets arrive at a consistent rate in 100 ms, doing the math, that’s 12 Mbps, which should not hit any threshold, either on a router or a switch. So no prioritization will take place.
So a statement like “if this type of packet comes in, it needs to be put on the wire ahead of any other packets that are waiting” assumes there are packets waiting. If thresholds are not met, packets are simply not waiting. Does that make sense? The question becomes “when are packets waiting?” For routers, it is usually when you get to the max throughput of the interface in question. For switches, it is likely less than the max throughput, for the reasons explained in the notes, but some threshold exists.
For such nuanced behaviors, it’s difficult to find documentation that explains such details. I believe that getting hands-on experience and seeing how devices behave in each situation is the best solution to understanding these features.
I hope this has been helpful!
Laz