Thanks for sharing.
In the previous description, you mentioned you will disable low power mode before reading FIFO.
Actually, in the low power mode, if you want to read FIFO, you need either enable "fifo_self_wakeup" bit or switch to sensor to normal mode.
When you want to switch sensor to normal mode, you need to wait 250us. Those might be the reason you need longer time here in 400KHz bus speed.
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