Confirmed on 0.99.109.1.6-beta (WebKit 613.1.12)
Only in vertical tabs mode. Does not seem to affect using the plus sign to make a new tab, I can quickly move cursor to the plus sign when it moves due to the tab inserting itself in the list, and click and continue making tabs much faster than if I was double-clicking.
If I double-click and continue clicking quickly, I get one initial new tab, and then no new tabs until I pause for about one second. Then I can make new tabs.