Using a modified version of my basic example of ESP-NOW usage, I tested latency and reliability of this networking strategy. 

The basic setup was that one device would send a message to another and start a timer. The receiving device would immediately send the message back to the original sender. Once the original device receives the echoed message, it stops the timer and records how long it took. An error counter is incremented if the message did not complete the round trip. I added padding bytes to the basic message format so that I could test sending messages of various sizes. I tested this for 1000 messages (sent at 5hz) with the two devices right next to each other on a desk (about 4 inches away), and with the devices across the room (about 128 inches away).

This testing was performed in my apartment in a city, so the 2.4GHz spectrum is very full. I used WiFi channel 1, and did not choose it intelligently (e.g. based on the power there.) I don't expect anyone else to be able to get the exact same results, but this can be used as a guideline.

The latency overall was good. The round trip latency is shown here, which includes some parsing/processing time. The latency for devices farther away seems to be slightly higher than for closer devices. The main concern with devices farther away seems to be the error rate. I expect this latency would be worse with encryption enabled.

The error rate seen in this testing is surprisingly high, and increases quickly as the devices are moved apart. Unidirectional error rate is shown here, which is the half missed messages (since this test uses a round trip approach). If you plan to send important messages using ESP-NOW, you may want to incorporate some resend-until-confirmed-received setup to your firmware. Direct messaging instead of broadcasting may be more reliable, but that was not tested.