Holding a snowball or harness and circling the Happy Ghast causes it to lag.
Steps:
Release a Happy Ghast.
Hold a snowball or harness to attract it.
Circle the Happy Ghast at a steady speed.
The Happy Ghast will keep looking at you but stops briefly during rotation. When you move behind it, its head snaps around suddenly.
Note: The same applies to small ghasts, though less noticeably. The Happy Ghast was tethered to prevent wandering. Pigs were used as a comparison (older mob) and showed no such issue—their heads rotate smoothly.
Environment
This issue has existed since the Happy Ghast was added.
When a boat is tied beneath the Happy Ghast, the boat's rotation remains smooth.
The Bedrock Edition does not have this problem.
No idea why, it’s probably client-sided lag.