Well my fan spins up now even when I have Discord’s web app open in Safari - it’s a React app 
I still think Apple are throttling older Macs now - when I first got this one it was amazing, apps opened instantly, fans rarely came on (tho fans rarely come on now too mind - sans Discord!) but now it takes what feels like forever to open apps (but maybe that’s cos they still haven’t fixed it calling home before launching apps) and just doesn’t feel like what it used to feel like - and there’s no reason why it shouldn’t be as snappy as it was given its specs. I can only conclude that either Apple have become extremely careless (unlikely given they are the richest company in the world) or they are purposely throttling older Macs because they want Mac owners to become more like iPhone owners - to upgrade every couple of years. It’s really making me go off Apple 