As usual, this is simply not true in general. If your system is doing 10 other things and is memory/CPU constrained then, sure, you probably know why it's taking so long.

If you have 100 tabs open of which 10 are rather badly-behaved, maybe the browser cannot do enough to mitigate it. But again, although this is a common power-user setup, it's not standard.

If none of the above, it's likely due to an extension. I was starting to blame Safari for slow tab opening and page loading recently and then I uninstalled an extension and everything became lightning fast again. I have around 100 tabs open across 4 windows. All major browsers are very fast for normal browsing in the default configuration.