@Vivaldi Thanks for the reply.
I am using a translator so it may not be accurate English. Sorry.
I am using the desktop version.
When I launched the Vivaldi browser and opened a window, then did something random (e.g. clicked on the search bar), a large number of tabs loaded in the background at a great speed.
After a few days, I launched it again and it started working properly, so that solved the problem for now.