I've also been having issues with webpages not loading or taking a long time to load in Vivaldi. I use a MacBook pro and MacBook Air. After much trial and error, (very much trial and error) I've come to realize that it's the font settings in Dark Reader that is interfering with Vivaldi on my Macs. I don't think it's an issue on my Windows machines. I don't use them much anymore. I've been on my MacBook Pro all day today and have had no issues with Dark Reader turned off.