If you have a 4096x4096 jpg image of approximately 10Mo, and you reduce its quality without changing the number of pixel, just in order to reduce the file weight (down to 2 Mo), will it help users to have fastest renders or less memory usage? Or does it depend only on the number of pixel of the image and not at all on the size in Mo of the file?
File size really only affects disk space usage, and very marginally render start-up time as a larger file will require more I/O to read. But that startup time is typically dwarfed by the actual render time, so the one-time start-up cost can be ignored.
When the renderer reads the image into memory, it is decompressed and stored at raw resolution and bit depth. So it will always use the same amount of memory for a given pixel resolution and bit depth, regardless of the size of the input file. E.g., an 8-bit RGB 4096x4096 texture will always use 49152MB of memory.