When I upload a category banner image my filesize of the image increases 5x from the original size. We optimize all images before uploading and would like magento to just use the file as is. Is there a way to prevent magento from resizing the category image?
If you mean category image, magento upload as is and use as is, file size shouldn't be increased anyhow. Make sure you don't have custom code or image optimization plugin that actually modified the original image
Any idea where to look? ie: where is this image saved at or what event may be involved? I don't have an extension installed specifically to do anything with images.
I would actually recommend using the image optimization module for this to avoid any issues with resizing and so one.
WebP is a modern file format smaller than JPG and PNG. So it takes less time to load and correspondingly doesn't slow down your website.
You could use Magento 2 WebP Images extension in order to convert all current and future images on your website to WebP format.