If you compare this to Windows 95 where the interface is only loosely defined, if at all, you'll see a big advantage since you don't have to guess. It is true that this separation initially caused problems. This competition, on the other hand, helped move the whole system forward in a short time. Those issues are now resolved and the Motif GUI style is the de facto standard. First, to make our product as stable as possible and comply with the X-Protocol specification, which is quite strict about what should and should not appear on the screen.
Speed comes in second.
If there is a trade-off between speed and correctness, we do sacrifice speed. This does not mean our drivers are slower. It just means they could be even faster if we would ignore some hardware problems of the graphics devices. Obviously, we want to have a fair benchmark that represents a real world scenario, rather than a meaningless number. With new and faster hardware, older benchmarks that gave meaningful results in 1989 are not appropriate anymore.