How to avoid loading low-detail imagery in Cesium? - cesiumjs

Question

Background
I'm using Cesium to run a simulation while capturing images with Viewer.canvas.toDataURL. The simulation has some action in it, and I would like the map imagery to be completely loaded when the action starts. However, for the first few renders, the imagery is always very low detail.
I only want to capture images at 15fps, so I render at most 15fps, for the sake of efficiency. The problem is that imagery starts low-detail and only updates when I render. It takes around 75 renders for the high-detail imagery to load, meaning that the first ~5 seconds of output images are rendered with low-detail imagery.
Question
I would like to reduce the number of output images with low-detail imagery to less than 15, ideally 0.
How can I
a) pre-load the imagery so that it's completely loaded by the time I start rendering
or
b) use the time in between renders to load the imagery, so that I minimize the number of frames with low-detail imagery?
Or maybe there is another solution I haven't thought of yet.
Things I've tried
1) I've tried restricting the area of the imagery provider, to load fewer images. This causes no discernible change, and in fact it does not cause any change to Viewer.imageryLayers._layers[0]._imageryProvider.rectangle.
var Viewer = new Cesium.Viewer('cesiumContainer', {
imageryProvider: Cesium.BingMapsImageryProvider({
url: 'https://dev.virtualearth.net',
rectangle: Cesium.Rectangle.fromDegrees(
Params.Lon - 5, Params.Lat - 5, Params.Lon + 5, Params.Lat - 5)
})
2) I've tried raising the minimumLevel of the imagery provider, hoping that would prevent it from loading the low-detail imagery at all. Again, this doesn't fix the problem and Viewer.imageryLayers._layers[0]._imageryProvider.minimumLevel is unaffected, always 0.
var Viewer = new Cesium.Viewer('cesiumContainer', {
imageryProvider: Cesium.BingMapsImageryProvider({
url: 'https://dev.virtualearth.net',
minimumLevel: 15
})
});
Minimal working example
var Viewer = new Cesium.Viewer('cesiumContainer')
Viewer.useDefaultRenderLoop = false;
var CameraTarget = new Cesium.ConstantPositionProperty(
new Cesium.Cartesian3(
-1673357.9867530654,
-4597650.657420824,
4077993.9927027463
)
);
var CameraOffset = new Cesium.ConstantPositionProperty(
new Cesium.Cartesian3(20, 2, 15)
);
var RenderInterval = 1 / 15; // seconds
var TimeDelta = 0;
var LastRenderTime = 0;
function ScreenCapture()
{
console.log('capture');
}
function Render()
{
var CurrentTime = Viewer.clock.currentTime.secondsOfDay;
var TimeDelta = CurrentTime - LastRenderTime;
if (TimeDelta > RenderInterval)
{
Viewer.camera.lookAt(
CameraTarget.getValue(Viewer.clock.currentTime),
CameraOffset.getValue(Viewer.clock.currentTime)
);
Viewer.render();
LastRenderTime = CurrentTime;
}
else
{
Viewer.clock.tick();
}
Cesium.requestAnimationFrame(Render);
}
Cesium.requestAnimationFrame(Render);