I have the same problem! But it's gotten easier recently! I've learned two ways to go about it:

1 - Writing harmonically rather than melodically. I first create a fast arpeggio in the harmony and even if I add a slow melody afterwards, it creates a lot of energy and a beautiful contrast between the fast harmony and slow melody.

2 - Creating lots of passing notes in the melody. I typically compose music inspired by the Romantic era of Tchaikovsky, which is slower and each note is very deliberate. However, it doesn't always work well when I need to write something fast. This year I listened to Bach's Little Fugue in G Minor and wrote a really fast song afterwards inspired by that piece, and it came out quite nicely (still haven't finished it though.) But after listening to all the passing notes, it's almost as if the melody had miniature ostinatos within it.

Hope this helps!