If the Amiga's mouse-pointer sprite was removed and the pointer was rendered onto a bitmap, it would still be 50Hz. There is more than enough time to do that.
The flawless refresh rate comes from the fact the update is executed as part of a level 3 priority CPU interrupt triggered by the vblank refresh. Interrupt code reads the hardware register of a dedicated chip that has been measuring the mouse's potentiometers this whole time, to see how much they've moved. The mouse itself is a passive device.
It then sends a input event to Intuition, whose priority 20 input.device task (the highest priority task in any normal Amiga system) processes the event and updates the hardware sprite coordinates, but could equally write it onto the bitmap. It has the time to do that because everything else in the system waits for it.
Compared to a modern PC, which also uses a hardware cursor... the mouse is an active device and sends its own potentiometer readings as USB input events, which arrive to be processed by an independent USB subsystem in the kernel and is bundled with all the other USB traffic from devices attached on the same bus. A vblank interrupt couldn't get the mouse coordinates even if it wanted to.
I should correct myself here and say it's not potentiometers, but that the mouse has vertical/horizontal wheels that let infrared light through to detectors, and the hardware is counting a quadrature-encoded pair of pulse trains, so it can tell how many steps back/forward each axis has moved.
It also estimates that, if the hardware counters are sampled every vblank, it's accurate (no missed wraparound) for the mouse moving at up to 3km/h! But it's still possible to whip the mouse around faster than that, so action games might like to sample the mouse more often than once per frame.
Apple loved doing things in software, ever since Woz wrote the Apple II disk routines in software so their floppy drives didn't need their own controller (the Commodore machines had a whole second CPU running an operating system in theirs). This is also why the one thing that WOULD lag the cursor on a classic Mac was formatting a floppy disk, since it required interrupt-level timing.
Even to this day Apple loves running things in software - the speakers on modern Mac laptops are driven in software - macOS has a daemon that runs and adjusts the speaker volume, tracking the sound pressure sent to them to keep them from being blown out. When Asahi Linux added speaker support, they had to reimplement this from scratch.
The flawless refresh rate comes from the fact the update is executed as part of a level 3 priority CPU interrupt triggered by the vblank refresh. Interrupt code reads the hardware register of a dedicated chip that has been measuring the mouse's potentiometers this whole time, to see how much they've moved. The mouse itself is a passive device.
It then sends a input event to Intuition, whose priority 20 input.device task (the highest priority task in any normal Amiga system) processes the event and updates the hardware sprite coordinates, but could equally write it onto the bitmap. It has the time to do that because everything else in the system waits for it.
Compared to a modern PC, which also uses a hardware cursor... the mouse is an active device and sends its own potentiometer readings as USB input events, which arrive to be processed by an independent USB subsystem in the kernel and is bundled with all the other USB traffic from devices attached on the same bus. A vblank interrupt couldn't get the mouse coordinates even if it wanted to.