A hybrid drive, if I understand correctly, is merely a standard hard disk drive - complete with moving heads and revolving platter - that's front-ended by a large (and sometimes non-volatile) cache. As such, you could perhaps expect its performance to be better than a 'regular' drive - but nowhere near what an SSD is capable of. And: the extent to which you'd see any performance improvement at all would depend entirely on how you were using the drive: the point of cache is to store regularly used data to make it accessible more quickly - but if you're not regularly using the same data, it won't do very much for you. Given all of that, I'm not surprised that the WEI is still just 5.9.
Surface Pro (2017) vs Surface Pro 4