Whirl yogurt, chopped cucumbers, herbs, lemon juice, 3/4 cup ice cubes, and the salt in a blender until smooth.
Pour cucumber purée into 4 tall glasses (12-oz. size). Fill glasses with club soda and stir. Add more ice if you like, and garnish with cucumber slices.
Make ahead: Through step 1, up to 4 hours, chilled airtight; stir before using.