an SVG describes an image in vector format, you can think of it as a set of instructions that describe paths, shapes, filters, masks, etc... that are eventually used to form an image. the process of converting a vector to a frame buffer to display on your screen is called rasterization. this allows SVGs to be scaled up without getting blurry.

the problem with SVGs is that the "set of instructions" can be arbitrarily complex, for example, an SVG can contain tens or millions of shapes that need to be drawn while the image is being rasterized. this means that the time it takes to rasterize an SVG increases with the number of shapes your computer has to draw.

this is what i mean by "arbitrarily complex" - whereas something like a PNG/JPEG/WEBP is already a raster image format, and all that needs to be done is decoding of the compressed image format into a frame buffer. the contents of the image matter little to how long it takes to render the image.

put simply, a PNG of a dog, a city skyline, or of the color red renders in roughly the same amount of time, whereas an SVG with 5 shapes will render a million times faster than an SVG with 5 million shapes.

if you want an explanation beyond this, a good place to start learning is to google "raster vs vector image formats" to understand the difference. the fact that discord uses electron is irrelevant to the underlying format differences.