There are some high-level optimizations that can be done in Lua, that just aren’t practical to implement in C++. An ordinary dev shouldn’t be expected to write their own animation system, but there is so much more that can be done when using a custom animation engine.
Level of Detail
My Lua Joints only exist and run when a graphic or sound is requesting them. When the hand parts are swapped out for a low detail version, the skeleton has fewer joints and parts to worry about.
If a character is off-screen or obstructed, it will stop updating and unload, resulting in 0 animation overhead. This optimization can’t be done in the roblox animation system because a script might want to know where the parts are.
Additional assemblies and animation types that sync by using the same animation ‘clock’
I can add more joints too, and sync their animations. I can also sync it with non-joint animations that play sounds, or anything else I want to make.

If it doesn’t move, it doesn’t update
I can set joints to have a static cframe or give them an abitrary Lua function that returns a cframe. This allows my Lua joints to fall asleep.
http://i.imgur.com/IzKmpTh.gif
http://i.imgur.com/4hDJWvc.gif
I can have joints without parts
http://i.imgur.com/DjjhDfA.gif
It can run any type of animation.
Animated hair, animated weapons, mounts, inverse kinematics; anything that results in a cframe, I can do it
http://i.imgur.com/0KIA5Gv.gif
Custom easing styles
My animation systems have had easing styles since before roblox even had an animation system. If I can write it in lua, I can make it an easing style.
2 keyframes:
http://i.imgur.com/FruWWKC.gif
(Groups of joints reuse the same easing result.)
Cache animation results
Once it’s looped through the same transition once or twice, it begins using old results instead of interpolating the same cframes.
Custom animation format and replication
If two completely different animations reference the same cframe, it only replicates that cframe once.
Transform Animation Data
I can mirror animations, or apply any other transformation I might need.

Real-time independent joint scaling

Reliable and Future Proof
Animations that use anchored parts and .CFrame are extremely stable and portable. I can run them in studio without relying on physics or humanoid animation controllers. I haven’t touched this animation system in years and it still works:
http://i.imgur.com/yCbXnbR.gif
C++ runs objectively faster than lua, but custom animation systems are more efficient for me as a developer.