Notification Center with Desktop Widgets consumes high CPU

Think hard about how much you want the Time in every listed city, updated to the millisecond, one screen-refresh away.

If you can get away with a slightly slower update, you could discard that widget and get back that processing power for things you care about.

The clock app is in the /Applications folder. If you place its Icon in the dock, it will display the up-to-date analog clock in its icon. You will be one click away from its additional features without using much processing power.

Calendar app is also in the /Applications folder. if you place its Icon in the dock, it was Icon will show the current date. I use this feature, and moved the calendar Icon to the Important end of the dock for quick reference. I don't use the rest of Calendar.

in the Menubar, clicking the time opens widgets, but you said you hate the amount of compute time the widget uses when not active.

In the Dock, clicking the clock icon opens clock.app anew, and gives you access to all those time zones with essentially no ongoing CPU used up.