Suddenly Phaser uses (almost) 100% GPU, crippling everything else

Phaser 3
1

After 2 years of good behavior my Phaser 3 game has suddenly changed.
As soon as new Phaser.game() runs, GPU usage jumps to nearly 100% (even though idle)
and the computer is burdened so that typing in devtools (or elsewhere) is ridiculously unresponsive.

This occurs on multiple (identical) computers
I do not think it is related to any recent code change
but I now init the game in a Promise chain, after loading level design files.
I doubt this is the problem but here is that code

Promise.all(  currentRound.levels.map( name=> 
            fetch( `./levels/${name}.json` )
            .then( res => res.json() )
            ))
    .then(  dbs=> dbs.forEach( db=>{ 
        console.log("SCENELIST", db);
        let cfg =  { key: db.scene.name, active:false, visible:true, db:db  };
        let next = null;
        switch ( db.scene.type.toLowerCase()){
            case "cut"    :  next = new CutScene(    cfg ); break;
            case "chase"  :  next = new ChaseScene(  cfg ); break;
            case "choose" :  next = new ChooseScene( cfg ); break;
            default       :  next = new BaseScene(   cfg ); break;
            } 
        sceneList.push( next );
        console.log("SCENELIST", sceneList); 
        }))
        
    .then (  ()=>
        game = new Phaser.Game(  {
                type: Phaser.AUTO,
                backgroundColor:  0xFF77AA,// 0x27176b, //rooftop may 16=  0x9aabe3,//0x034047,// 0x3254e7,
                width: 9000,
                height:800,  
                scale: {
                    parent: 'gamescreen',
                    mode: Phaser.Scale.FILL,
                    width: 1920,
                    height: 1080
                    },
                parent: 'gamescreen',
                dom: {  
                    createContainer: false 
                    },
                physics: {
                    default: 'arcade',
                    arcade: {
                        gravity: { y:1300 },
                        debug: dbg.seePhysics                  
                    }
                },
                scene: sceneList,
                plugins: {
                    global: [
                        { key: 'SceneWatcher', plugin: PhaserSceneWatcherPlugin, start: true, mapping: 'sceneWatcher' }
                        ]
                    },
                callbacks: {
                    postBoot: function (game) {      game.plugins.get('SceneWatcher').watchAll();  }
                    }
 
            })    )
        ;