We can’t modify the Happy Ghast speed with the fly speed and movement speed attributes unlike other flying mobs.
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/attribute @n[type=happy_ghast] minecraft:flying_speed base set 1
/attribute @n[type=happy_ghast] minecraft:movement_speed base set 1
These commands are supposed to change the Happy Ghast speed but they don’t (this may be intentional but would be a great feature to add)

Confirmed. Setting both attributes to 0 doesn’t make any difference.
Might duplicate MC-40949, but that report is very generalized.
Going to resolve this as a duplicate of MC-40949
Please provide the command needed to reproduce this issue