Any javascript reach website opened in the side-view abnormally increases Firefox's CPU usage as compared to the CPU usage the same website opened in the main tab.
For example, the web-version of https://www.messenger.com/ takes around 8-10% CPU, but, if I open this in side-view, it immediately goes up to 120% CPU usage.
Platform: MacOS 10.15.5 Hardware: MacBook Pro (15-inch, 2018), 2.6 GHz 6-Core Intel Core i7, 16 GB 2400 MHz DDR4 Firefox: 77.0.1 (64-bit)
Any javascript reach website opened in the side-view abnormally increases Firefox's CPU usage as compared to the CPU usage the same website opened in the main tab.