I am looking for a way to adjust a player's (or entity maybe) movement speed based on the material they are walking on (and also if they are colliding with something like tall grass)  I have had a sort of success with capabilites.movementspeed and the Player.Tick event, but I feel like the tick event is too intermittent.  I need a better way to instantly change the speed as the player moves.

Unfortunately, you can't get much more instantaneous than every single tick in Minecraft.

 

Tick events have a Phase.START and Phase.END, so they happen twice per tick. Use the START phase to change movement speed so it affects the player's movement in that particular update tick, or during the END phase to change it for the next tick.

 

To make the code nice, you could make a Map of Block Material type to movement speed modifier, then add and remove AttributeModifiers for SharedMonsterAttribute.movementSpeed to the player:

public static final UUID blockMovementModifierUUID = UUID.fromString("PutSomethingInHere: see http://createguid.com/");

// every tick during Phase.START:
IAttributeInstance movement = player.getEntityAttribute(SharedMonsterAttributes.movementSpeed);
// remove old:
AttributeModifier old = getModifier(blockMovementModifierUUID);
if (old != null) {
  movement.removeModifier(old);
}
// Add new:
double value = yourMap.get(currentBlock.getMaterial()); // one should check if map contains value first
// Attribute Modifiers have 3 [url=http://minecraft.gamepedia.com/Attributes#Operations]operations[/url]: 0 - additive, 1 - multiplicative, 2 - 1+value multiplicative
AttributeModifier modifier = (new AttributeModifier(blockMovementModifierUUID, "Block Movement Modifier", value, 1)).setSaved(false);
movement.applyModifier(modifier);

Thank you, that was very helpful.  I will give this a try