Forge 47.3.0
Abnormals Delight 5.0.0
Neapolitan 5.0.0

Basically, vanilla_cake_slice doesn't give the consumer the Speed effect when eaten, despite its code attempting to give the consumer speed. (At least, I think it does, since the cake slice automatically* appears under minecraft:speed when Just Enough Effect Descriptions is installed. Refer to the screenshot below.) I believe this is a result of Neapolitan's Vanilla Scent effect gained when eating the cake.

Has not been tested on a minimal instance.