I've always wondered whether network interfaces that have these flashing lights flash as a gimmick or do they actually indicate the flow of traffic? Perhaps one flash per packet in or out? I wish I could remember what my call up modem looked like to make a historical comparison too.
TL-SG105E
I'm going off what I remember from a decade ago when working on embedded CPUs that have an Ethernet interface. IIRC, the activity LED -- whether a separate LED than the link LED, or combined as a single LED -- is typically wired to the PHY (the chip which converts analog signals on the wire/fibre into logical bits), as part of its transceiver functions. But some transceivers use a mechanism separate from the typical interface (eg SGMII) to the MAC (the chip which understands Ethernet frames; may be integrated into the PHY, or integrated into the CPU SoC). That auxiliary interface would allow the MAC to dictate what the LED should indicate.
In either case, there isn't really a prescribed algorithm for what level of activity should warrant faster blinking, and certainly not any de facto standard between switch and NIC manufacturers. But generally, there will be something like 4 different "speeds" of blinking, based on whatever criteria the designers chose to use
Your knowledge on this is amazing and I wish I also worked with such this earlier on. Thanks so much for sharing! :)