Create MIDI instrument with 52 inputs and low delay
How about an array of Arduinos? It won't be the smallest solution, but it should still be reasonably cheap and easy to develop.

Depending on the model you pick, each board will have between 6 to 16 ADC channel. These will be connected to an individual xylophone key. You then write code for the Arduinos to handle their own keys. Since this is the only thing the Arduino has to do, it should be feasible even with the limited CPU power.

You can then put these on a common SPI or I²C bus, talking to either another Arduino, or a Raspberry Pi, that will convert the individual messages to a MIDI stream.