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Many 'big operations' will use some sort of image-resizer CDN, whether something custom made in-house, off-the-shelf/open source (like thumbor), or a CDN like Imgix.

To prevent anyone from fiddling with the URL to generate thousands of differently sized images, they'll often have a way to 'sign' the URL to ensure people can't do this sort of attack. This can be expensive, depending on how its done I guess.

It could be a hash operation, yes. But would it really be necessary to make it a cryptographic hash? It would explain the performance issues, yes. But who uses cryptographically-secure hashing for use-cases where the answer is "MD5"?

>I think it's only loading the URL to the profile photo image

Sorry, by "it" I should have written "the client." I meant to suggest my question of whether that is where their client OOM errors are coming from.

We load a small thumbnail; it's the extra memory allocated to strings that was the main factor that helped when we reduced the # of comments sent down.