While yes, you can saturate a 1GB link using the Pi 4, the 4x A72 core complex is not going to be blisteringly fast no matter how you slice it. If you're trying to stream encrypt with AES, you're going to be limited to at best half link speed since since the A72 manages only about ~50MB/s. This, of course, isn't just an encryption issue but really just that once you factor in everything else that it takes to host a complex software package like Mastodon, it's hardly surprising that a Pi 4 struggles under the weight of both HN and the rest of the fediverse slamming the tiny SoC.