Idle CPU Usage Question

Right off the bat, 20% sounds like a lot for a computer at idle, but it really depends on many things, It's not an easy question to answer, it may be normal in your case, or it may not.

Anything you have opened contributes to background usage, even if it is notepad for example, which basically does nothing until you type on it right? Just because it is opened, the scheduler (the part of the OS that says whose turn it is to run) will be giving it some time at intervals.

The more elaborate the software is, the more it contributes to background CPU usage. For example you named a web browser, that one is much more powerful than a simple notepad and having a single tab opened could be enough for the usage to go up by a noticeable amount, specially if there are animations of the tab, or video, or audio, etc.

So yes, VPNs do increase CPU usage, just like any other program. Without the VPN, traffic follows a route which we're going to simplify like hell and say it goes from the App -> OS -> Network Card -> Internet. With a VPN enabled however that traffic needs to be intercepted at some point in that route, hence the increase in processing and increase in CPU usage.

As for the number, I said 20% sounded like a lot, but it really depends on how powerful your processor is. For example, simplifying things a lot again, say you have just 1 core running at 1 GHz, and you see a 20% at idle with whatever is opened in the background. If a similar machine, but equipped with a processor that also had 1 core, but run at 2 GHz instead you will not be seeing a 20% in there, but a smaller figure. And the same happens as the number of cores increase.

It's not linear, you can't assume for example, 1 core -> 20%, 2 cores -> 10%; but it does affect the overall idle consumption.

You could open the Task Manager (Ctrl+Shift+Esc) and check the details tab for the process(es) that are using more CPU cycles when you're actively doing nothing with the computer.