Awesome UX. I believe this implementation is called a v-buffer, though I can't find any references right now but I am fairly certain I read about it a decade back.
Apple used this technique for their menus early on while Microsoft took the alternate route of using a delay which was quite annoying. A lot of power users removed the delay via registry settings and it was surprising why Microsoft didn't adopt the v-buffer--might be patents related?
The problem with the delay is not only the delay itself but for wider menus the delay needs to be even longer. Then to account for novice users, you add in a little bit extra just for good measure.
One way to mitigate long delays is to make the parent menu taller so the user has a bigger margin of error while they move the mouse horizontally to the right.
However before hi-res displays became common, vertical real estate was far more limited. On the earlier windows machines where you often ended up having two columns of menus, this was even more annoying because trying to get to the second column of menus required moving your mouse right, through the first-column's submenus. In later version of Windows, Microsoft did away with two column menus and introduced a single column with scroll up/down to solve this issue,but introduced further delay when trying to get to menu items below the visible screen.
(edited for grammer)