Here is a list of tips on how to avoid certain things which may cause your game to run slowly in the browser.
Dynamic Color Changing
Since E2C doesn't use WebGL, changing the color of a SpriteRenderer dynamically is a heavy operation. It requires an offscreen <canvas> to be created, the whole sprite to be drawn on it, then tinted, then drawn again on top to restore alpha channels. Changing the color of a SpriteRenderer every frame will require all of these operations to be performed every frame, which will be very detrimental to your framerate. You can avoid it by minimising how often you change the color (changing it once and then leaving it is okay). You can also completely avoid it by having pre-colored sprites that you change dynamically, instead of changing the color.
If you just want to change the a value of the color, that will not require tinting and will perform just fine, even if you do it every frame.
This applies for SpriteRenderer, TextMesh, UI.Image, UI.RawImage, UI.Text, MeshRenderer.
Using UI.Mask will potentially reduce the framerate. Whenever possible you can instead use UI.RectMask2D which will perform slightly better, but if you can avoid using masks altogether that would be best.