IORD_ALTERA_AVALON_PIO_DATA to slow?

Sure, you must in every case read the fifo faster then the values being written into. More exactly, you must be faster on a average. The fifo serves only as a buffer which avoids losing data samples.

The advantage is that you must not continuosly poll the PIO to get data samples synchromously to your adc source, but you can allow data to accumulate into the fifo; then you can read bursts of n values, which is usually a faster process than reading a single data n times.

You can conveniently generate the fifo component with the used word port feature and then periodically launch a dma to read the exact number of stored samples.