It is true that the time to fetch, render, and deliver an image the first time it is requested can be a bit longer, because of all the processing we're doing in the case of that single request, but because we then cache that fetched image, all subsequent requests are delivered an order of magnitude faster (50th percentile 45ms).
The majority of the time taken in the first request is actually in fetching the image from the origin source, so once it's cached in our system it becomes a much faster operation: and of course delivering the cached image without it traversing our stack is even faster.
So yes, the initial request can take time, but all of the subsequent requests of that image are much faster than the alternative. And when you take into account that our service makes it possible to send the correctly sized image for the display (instead of loading a preset size and displaying it smaller), and optimal file types based on device and browser (webp for chrome, etc), load times/page weights on all of those requests are significantly improved.
In general, anyone who serves multiple requests for their images over time will see a marked improvement on page weight/speed, compared to rolling their own solution where they have preset image sizes and deliver jpeg only.