Hardware acceleration is a great example.
Opera’s rendering was so fast the other browsers had to use hardware acceleration to catch up. Because of Opera’s superior rendering performance hardware acceleration hasn’t really been necessary. Only now there are some better use cases for hardware acceleration, and lo and behold, Opera is on it.
You are only confirming the point that browsers have different priorities. Opera’s priority has obviously not been to rush out hardware acceleration since the software rendering is already very fast.
Not slow development, but rather a matter of prioritizing development resources.