Yea, well, the CPU spike isn't only for this browser, it appears on all of them, including Chrome. It is lesser, sure, but it is present there nonetheless. And I'm saying this after also trying it by having having simultaneously opened 20-30 tabs - Chrome uses 6-9% CPU (one tab playing YouTube video in background), yet the moment I begin reading a chapter in Chrome is spikes to 9-15%. So no, this bug isn't Edge specific. How could reading text use more resources than playing a video?
Also, to receive a CPU spike, you don't need to scroll over multiple chapters, directly opening any one of them will cause it immediately.
Normal state of CPU when having multiple webnovel pages opened without opening any Chapter to read

Spike on CPU after opening a chapter to read, without scrolling down and opening multiple chapters

Here I have Chrome opened with 8 additional tabs, with the 9th being Webnovel.com Library:

And here is Chrome with Webnovel Library being replaced with Webnovel Chapter/Reading (after the CPU stops fluctuating)

A spike of 2.3% for what? Each tab in Chrome on the first page uses 0.24% (2.2% / 9 tabs), yet the only difference in the second image is Webnovel Library changing to Webnovel Chapter/reading page and CPU usage jumps from 2.2% to 4.5%. That is more CPU usage than all 9 tabs from the first example combined.
Also the Chrome spike even after stabilizing still fluctuates between 4.4% and 5.6%, which means I took a screenshot of nearly minimum for the above example (I've been writing this post in Edge while Chrome was opened in the background all this time without scrolling down the Chapter or even using it at all, so that's not the reason for the fluctuations!)