IORD_ALTERA_AVALON_PIO_DATA to slow?

You probably want to put a buffer inside your ADC avalon slave. That way the ADC gets guaranteed write access times (M9K buffers are dual ported).

You then need to add logic to your slave interface to access that memory block.

It might be possible to leave space before the area where the ADC writes for an ethernet+IP+UDP header - and then transmit directly from that buffer!

That would be a 1k+ UDP packet containing 512 16bit samples. Double buffer and a nios could schedule the transmits.

Of course if you are using a network stack it all gets harder :-)