Whenever you are having problems with a passage with many steady, fast notes, concentrate on getting the first of the group on the beat - and the others will follow. This is an example of the macro-rhythm concept. So if you were playing a sixteenth note passage, say with 4 measures of 16th notes, and it was too fast, messy, not fitting in the time you've got, concentrate on getting the first of each group of four on the beat each time. This will get you in the ballpark as the other notes get into place.
Complex chord structure creates musical sophistication. Yet the opposite is not true - simple chords = sincerity, not simplicity.