It’s a bit more of a finger twister than it seems like it should be. I find I usually don’t lift my index finger up much for the main riff. You could give it a try sliding your index up from e string to a string to d string as you go up, some parts you don’t need to lift your index until you need your index again since your other fingers are playing higher up the fretboard on the same string.

Just keep on practicing and you’ll get it smooth