Great in-depth writeup on how Discord struggled to resizing lots of images via their Image Proxy Service:
As Discord grew, the Image Proxy started to show signs of strain. The biggest problem was the Image Proxy did not have an even workload distribution, which hampered its throughput. Image proxying requests saw a wide variance of response times, with some taking multiple seconds to complete.
Eventually they created Lilliput, to resize a scale of images and generating first-frame stills of video:
Lilliput relies on mature, high-performance C libraries to do most of the work of decompressing, resizing and compressing images. It aims to do as little memory allocation as possible and especially not to create garbage in Go. As a result, it is suitable for very high throughput image resizing services.
Lilliput supports resizing JPEG, PNG, WEBP and animated GIFs. It can also convert formats. Lilliput also has some support for getting the first frame from MOV and WEBM videos.