Too many devices using the same bandwidth at once.
Several factors can contribute to jitter in your internet connection: speed test jitter
In technical terms, jitter is measured as the difference in milliseconds (ms) between the arrival times of consecutive packets of data. A lower jitter value indicates a more stable and consistent internet connection, while a higher value indicates a more variable and potentially problematic connection. Too many devices using the same bandwidth at once
If your router is more than 3-4 years old, it may struggle to manage modern high-speed data packets efficiently. speed test jitter