Two nRF24L01 radios per Arduino
At 2 Mbps, a 32-byte packet (40 bytes with preamble, addresses, CRC) will be approx 160 microseconds long.  The round-trip time is likely to be much longer, since the microcontroller or radio will need additional time delays for auto-retry, sending and receiving the response packet, PLL lock, reading the data registers, etc.  In my experience a 2 ms round trip is possible, and the best way to do that is make use of the nRF built-in features:

-- IRQ interrupt: interrupts = fast

-- Enhanced Shockburst: this automatically handles lost packets and failed transmissions.  You can't get that capability any faster via the microcontroller coding.

On top of this, there is scope to use fast SPI libraries (or use SPDR/SPCR registers directly) to transfer data to and from the radio module.  If speed is your goal, don't rely on plug-and-play libraries that others have written.  Read the data sheets carefully and minimize unnecessary delays within the Arduino code.