I just did some tests and was unable to replicate your results. I'm on an i5 2019 Mac, and Vivaldi takes about 200ms to open a tab, with or without CSS filters.

I just did some tests and was unable to replicate your results. I'm on an i5 2019 Mac, and Vivaldi takes about 200ms to open a tab, with or without CSS filters. (200ms is bearable, but it is slower than I would like.) I have Hardware Acceleration on and Use Animation off.