@leocg Hi, I have got the same issue. The abnormal CPU usage starts after receiving some messages (also in the background). I have tested the side panel window and normal tab. Both started draining my CPU after some while.
Usually, it helps me reload the tab, but after some time, the high CPU usage started again.

System info:

• Apple M1 Pro
  • Version 15.3.1 (24D70)
• Opera GX
  • version: 117.0.5408.100