This is a known issue, commonly (mis)named the "I218-V Wake On LAN broken with Fastboot bug" - it was broken on Win7,
it was broken on Win8 and 8.1,
now it's broken on Win10. "Wake On LAN" should probably be renamed to something like "Fails On Boot" or "Fails To Properly Reset/Initialize On Power Up", but it's not.
Your I218-V slowdown problem is not uncommon. Other common symptoms include random disconnects, random failed connects, excessive lost packets, failed checksums and error-corrections, failed modes, failed duplexes, failed teaming, etc. Intel hasn't admitted any specific details and always says that they're "still collecting data". But there's been much "expert" speculation and analysis by (non-Intel) hard-techs pointing at certain flaws in the I218-V PHY (electrical interface) implementation - these are supported by "conclusive" and reproducible tests demonstrating that I218-V parts have oversensitive response to momentary power fluctuations which do not meet minimum advertised fault tolerances, and by inconclusive tests showing that variance in individual I218-V parts often fall below acceptable reliability/performance thresholds. (Remember that the I218 family was primarily designed to integrate with Xeon-based Cxxx chipsets, it seems Intel may have cut them down a little too far for their Core-based Express chipsets. I honestly don't know if Intel or Asus bins these or any other chipset components - you'd expect only the best and fastest in your ROG, yes? - though I suspect that at least the majority of obviously below-spec or borderline parts get weeded out during QA testing phases.)
Intel eventually released drivers which implemented an internal workaround to magically fix all these I218-V issues (through some kind of forced IRQ reassignments or something) on Win7 and Win8/8.1, I expect they'll get around to doing the same for Win10. You'll always find the latest-and-greatest
I218-V drivers at the Intel Download Site first, even when they're not (yet) available on the
Asus ROG Rampage V Edition 10 download site.
The user comments on my links above offer a little advice about how to fix the issue, though no comment on whether or not any of the suggestions actually worked. You might be stuck living with your frustrating issue until a new Intel driver fixes it or you retrograde to an old Intel driver fix on Win7/8/8.1. Or you could always fallback to the I211-AT Gigabit controller, though it has plenty of frustrating bugs too.
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