This message was posted on behalf of Intel Corporation
The Intel® NUC NUC6CAYS was been officially released to work with Windows® 10, I am afraid, I do not have notifications about WOL issues with LibreELEC.
You may need to address this matter with the Linux community to see if they offer updated LAN drivers https://forum.libreelec.tv/
I did extensive research on the problem. You are not the only person who has experienced near-100% CPU load with Libreelec 8.0. It is not limited to NUCs only.
There seemed to be no conclusive cure for the lag either that people could recommend, as it seemed to be under different circumstances, not just after WOL like on yours. The majority reported that the lag occurred when playing HD videos. One person cured their lag by turning off subtitles on the videos.
I expanded the scope of the research by searching for 'wake on lan high cpu load'. It seems that Wake On LAN is a common cause of high CPU load, and that everyone who disabled that feature had their CPU usage drop hugely.
I'm not an expert on the eMMC storage, except that it is limited-capacity Flash-based storage. It is meant to be used to store a small thin-client OS, not for regular read-write use (as repeated writes degrade Flash storage, unlike SSD drives). So it's probable that the switch to SSD was indeed the correct solution for your particular NUC setup.