There are vocal functions that require excellent balance and flexibility that don’t involve lots of fast notes. These are the vocal equivalents of tight-rope walking, as opposed to trapeze. Slow exercises, such as the ones below, will help you get to the next level vocally. If they are already easy for you, they will help you to keep your voice fit for many years.
The messa di voce and its relatives, the crescendo and diminuendo:
The ability to do these things on a steady pitch with a steady sound (with vibrato or without, depending on the singer and the genre), to their dynamic extremes, takes a lot of practice, but will help with your expression, range, and vocal stamina.
How does the note begin? How does it end? Play with doing things to the middle, as well as keeping it absolutely constant. How can you make one note charming, or mystical, or alarming?
Vowel change on one note:
Making each vowel distinct from the rest, but also related to each other. Record and sing them for people. Are they sure what vowel you are singing, without words for context?
Singing different colors or timbres on one note:
This can suffer from abstraction. Many times musicians cannot agree on terms like “bright” and “dark”, so it might help to think of moods or expressions that you want to convey with sound. To what extent can you do this without text to sing? When you sing text, can you make the same sentence on the same pitches mean different things? Try it with both speaking and singing.
Slurring two pitches that are far apart:
Sing a few seconds on a pitch, go to the distant pitch, then return. Did the third note change in character from the first? If so, how is it different? Can you make it the same? When might you want it to be the same, and when might it change with a purpose? Try this in different parts of your voice, at varying volume levels.
Practicing fast scales, riffs, and ornaments is also important, but should be alternated with these slow skills to keep your voice fresh, flexible, and capable for many years.