Fixed it! I just went to the modules/MMM-PIR-Sensor directory and did

npm install

That fixed it.

Thanks v.much to everyone on this forum. I just downgraded Electron to 1.4.15, without downgrading either MagicMirror2 or my OS, and my CPU usage and temperature are significantly down. But I’m facing yet another problem because of this downgrading of Electron. My MMM-PIR-Sensor module is giving this error:

App threw an error during load
Error: Module version mismatch. Expected 50, got 54.
    at Error (native)
    at process.module.(anonymous function) [as dlopen] (ELECTRON_ASAR.js:173:20)
    at Object.Module._extensions..node (module.js:583:18)
    at Object.module.(anonymous function) [as .node] (ELECTRON_ASAR.js:173:20)
    at Module.load (module.js:473:32)
    at tryModuleLoad (module.js:432:12)
    at Function.Module._load (module.js:424:3)
    at Module.require (module.js:483:17)
    at require (internal/module.js:20:19)
    at bindings (/home/pi/MagicMirror/modules/MMM-PIR-Sensor/node_modules/bindings/bindings.js:81:44)
Whoops! There was an uncaught exception...
Error: Module version mismatch. Expected 50, got 54.
    at Error (native)
    at process.module.(anonymous function) [as dlopen] (ELECTRON_ASAR.js:173:20)
    at Object.Module._extensions..node (module.js:583:18)
    at Object.module.(anonymous function) [as .node] (ELECTRON_ASAR.js:173:20)
    at Module.load (module.js:473:32)
    at tryModuleLoad (module.js:432:12)
    at Function.Module._load (module.js:424:3)
    at Module.require (module.js:483:17)
    at require (internal/module.js:20:19)
    at bindings (/home/pi/MagicMirror/modules/MMM-PIR-Sensor/node_modules/bindings/bindings.js:81:44)

Any help on fixing this would be appreciated.