Is that still an issue with the c3 runtime? And if so, would you suggest leaving it on all the time?
It's the case for all Cordova apps, regardless of what runtime you use. Cordova builds hybrid apps, which Construct 3 uses to wrap HTML5 games for mobile.
I suggest leaving it on during gameplay. But when on the menu, for example, you can turn it off. The plugin lets you choose when to lock or unlock.
The ones throttling the performance are the device manufacturers, they filter out which to run their device's performance to its full extent and which to run idle mode, to save battery life.
I tested this myself on a few devices. And I got a huge performance boost during locking.
Not only that, if you wait for your application to reach a time that it goes idle mode when you don't use the plugin, it will have a huge FPS drop. Which is terrible for games especially realtime multiplayer.
Though, this a device-specific case, not all devices have this. But I assume most devices that promotes battery life have this trick, which device manufacturers call smart battery, which is a feature, they're not wrong. But, it throttles the performance of Cordova games.
Assuming for example 50% of devices have this 'smart battery' feature, you'd have 50% players not running your game on the full extent of their phone's hardware.
As much as possible, it's recommended to run your games to the full extent of a device's performance.
I had the slight concern that it would get rejected from the App Store if the screen was forced on. Sounds like that won't be the case.
Nope, it won't. And it helps that you can just turn it on when you need it, for example during in-game.