Just activate the piston, the frames will drop some times
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I'm sorry but i used the wrong word there.
The fps of the game doesn't changes, it's the piston's animation laggy

Not only that, but the block that gets moved by pistons as well. Sometimes the block moves pretty smoothly; sometimes the block just disappears out of the way.
It effects 1.16 latest beta.
Thank you for your report!
However, this issue has been temporarily resolved as Awaiting Response
Is this still an issue in the latest version?
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Hi.
Yes, the animation is, indeed, still laggy, but not always.
I play on both Xiaomi Redmi Note 8 (Android 10) and a Windows 10 machine: It's laggy on the phone and the block just teleports on the computer.
And it seems to be purely random as I was unable to notice a clear pattern to reproduce the issue.
As for what is expected to happen:
Obviously the block, that's being pushed/pulled by the piston/sticky piston, should move very smoothly so the players can see, clearly, where the block is going.
As for the version that is being affected: The latest full release of 1.16. I'm not sure, however, whether or not the newer versions with the Renderdragon engine enabled are affected or not.

Is this still an issue in the latest version? If so, can you add it to the affected versions?
Cleaning up old tickets: This ticket had been set to 'Awaiting Response', but has not received a response from the reporter (~3 months+) so is being closed as Incomplete. If you feel this is still a valid issue then please comment, or create a new ticket following the Issue Guidelines which includes steps to reproduce the problem.
For any account or purchasing related issues, please contact Minecraft Customer Support directly, as we cannot assist with those here at the bug tracker.
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Re-resolving as a duplicate of MCPE-155987, which has explained the issue better.
I am unsure as to what you mean when you say that frames drop. Do you mean that the piston causes a decrease in FPS? or something else?