It is truly showing overall usage.
You can find the same information, but with far more individual counters, in Performance Monitor (Run perfmon.exe) or Resource Monitor.
Keep in mind that the granularity in Task Manager is quite coarse - it is only updated once per second by default, once every half a second at most. For CPU usage each graphed point is essentially showing the average CPU usage in the preceding interval; spikes in usage can "fall between the cracks".
Also note that these tools do involve code running in at least one thread, so they affect the system being measured; you are not seeing what would be happening if you weren't looking. Usually this difference is insignificant, but it's there.
You appear to have hyperthreading enabled. You should know that that really only gives you four cores' worth of performance in the long term, though it can sometimes give you more than that in terms of responsiveness. The CPU graphs you are seeing of the Windows scheduler's normal behavior, which is to try to use only one logical pprocessor of a core at one time, thus not splitting the core's CPU power between two threads.
The overall CPU usage percentage shown there, "20%", in this case, is sort of misleading when hyperthreading is enabled. Suppose that only one LP out of each core was in use, each 100%. The overall usage would show 50%. Now suppose all logical processors were in use, each 100%. The overall usage would show 100% but you would not be getting twice as much work done as before. 40% more is about the best that can be expected.
Hyperthreading aside... when you are talking about responsiveness, it doesn't matter if there's 50% of your total CPU power available, or even 80%, as a long-term average. (In this business, an average over a one second interval is "long term".) What matters is if it's available right now. So the CPU graphs can be useful for figuring out the long term impact of CPU-intensive jobs, but not so much for understanding responsiveness issues. For that you probably want to look into the Windows Performance Toolkit.
When I see lags in terms of several seconds, and file opening delays of several minutes, this should show up in Task Manager, right? It doesn't seem to be--although during a recent file open that took several minutes there were intermittent peaks across all 4 CPUs (and not the hyperthreading cores) when I changed the update speed to "high." So perhaps it was intermittently maxing out the CPU but it's difficult to tell. – MacsAre1 – 2017-04-29T01:17:59.010
BTW, I can't install the Windows Performance Toolkit and getting IT to install anything for me is like pulling teeth. Need to have an understanding of what the performance needs are without having to install anything else. I can see that I'm lagging, and I can see that removing lines from my AutoCAD Civil 3D drawing helps, so I'm leaning towards memory being the current bottleneck. I do have a current i7 6700 CPU which should be OK for CAD. – MacsAre1 – 2017-04-29T01:24:30.587