Keeping the Tab key pressed is a common way to quickly navigate through focusable elements in a page when using only the keyboard. That's a native behavior and should work in any web application.

However, when keeping the Tab key pressed, there's a weird effect on some blocks where tabbing enters in a sort of "loop", I guess because of timings and the non-native focus management Gutenberg does.

This is particularly noticeable on the cover image block and in the image block, but I have been able to reproduce it randomly on other blocks too, for example the paragraph block.

To better illustrate the issue, I've made a short video (25 seconds) of what happens on current master: https://cloudup.com/c8n7UoQGBaZ