If a beacon gives you regen effect it will heal the player slower than normal regen effect.

How to replicate.

Change game difficulty to Hard
Give yourself hunger effect until you have no hunger bars.
/effect give ilt minecraft:hunger 8 88
Let your health drop below half
then give yourself regen effect
/effect give ilt minecraft:regeneration 888 0
Notice you will heal you health over time.

Compare to the regen that comes from a beacon, which is slower than the normal speed.

Change game difficulty to Hard
Give yourself hunger effect until you have no hunger bars.
/effect give ilt minecraft:hunger 8 88
Let your health drop below half
then build a regen beacon near you
Notice you will NOT heal you health over time.