Thanks for writing in.
It’s hard to say whether your observations are expected. Those things will always depend on one’s hardware. One thing is sure, given a fixed number of total data points, performance improves as the number of traces go down. In your case, (300 * 3000) = 9e5 should be ok for plotly.js if plotted in a single surface. Beyond that, our 3d renderer will probably start to feel sluggish on most hardware.
So in brief, you should try to combine your traces into one.
As for
I invite you to read Colors for discrete ranges in heatmaps - #4 by empet
where a solution is given for creating custom “discrete” colorscales (that example shows off heatmap traces, but should work for surface traces as well)