@yury I've never used Mysensors but looking at the latest RFM69 driver it's clear that it waits minimum MY_RFM69_CSMA_TIMEOUT_MS (default 500ms) before sending a frame.
https://github.com/mysensors/MySensors/blob/master/drivers/RFM69/new/RFM69_new.cpp line 378

I suppose it does it to respect the regulations.

EU regulation is as following:

Source and details here.
It is very clear that only 2 400 -2 483,5 MHz can be used for these kind of sensors. Higher frequencies are used by medical devices.

Hints on RS-485 networks:

Termination resistors
You always need them. Termination resistors should be added to the nodes located at the ends of the line. The communication may works without them if the wire is short enough and/or the bit rate is low.

Pull up/down resistors a.k.a failsafe bias resistors
Why you need it is well explained here: https://electronics.stackexchange.com/a/284788/88486
When you need it: It depends on the RS485 transceiver IC. Most modern transceivers include these.

Common ground
Do you have a common ground between your transceivers? RS-485 is not a 2 wire network. Besides the A-B lines it requires ground.
See: http://store.chipkin.com/articles/rs485-rs485-cables-why-you-need-3-wires-for-2-two-wire-rs485
Schematics at http://www.analog.com/media/en/technical-documentation/application-notes/AN-960.pdf page 4.

Isolation
When you deal with long links, you have to take care of isolation.
Page 8 at http://www.analog.com/media/en/technical-documentation/application-notes/AN-960.pdf

@boozz
It is pretty easy to use these docs. Just look up the frequency you use and check what are the limitations.

In the ECC document (page 9) you can find the followings for 2424MHz:
Max power: 10 mW e.i.r.p.
Spectrum access: No requirement

This means that you can transmit all day if you want (no requirement on spectrum access), however, the tx power may not exceed 10 mW e.i.r.p. nrf24L01's max output power is 0dBm which is 1mW so using this module you probably cannot violate the rules.

I did not mention it in my first post, but I'm more worried about the 868MHz band.
Same doc. page 8, row h1.5 868.7-869.2 MHz
Max power: 25 mW e.r.p.
Spectrum access: ≤ 0.1% duty cycle or LBT+AFA

For this band RFM modules like RFM69HW or RFM95HW are often used. The max output power of these devices is 100mW so if you just set the module to the max TX power, you are already way above the limit.

The trickier part is the 0.1% duty cycle which means your airtime per device cannot be more than 60ms per minute (86,4 seconds per day) unless your device implements LBT and AFA Sometimes this is hard to respect, especially in LoRa mode.

Hi All,

I'm not sure if this is the right place for the topic, but I thought it's worthwhile to come up with it somewhere.
I've looked around at MySensors and I have to say that the developers did a great job, it is obvious that they know what they are doing.

Unfortunately some of the users not so much. There are a lot of beginners who violate the corresponding regulations. Maybe it is not intentional, perhaps they don't even know that there are rules for the so called license-free bands, too.

For those who haven't heard about these yet:
I really don't want to be a party killer, but it is not as free as you think. There are both power and duty cycle limitations for each sub-band. You cannot transmit at +20dBm whenever and whatever you want because it is illegal, but most importantly because it can jam your fellow developers' devices in the neighbor or across the street. The licence-free frequencies are common and shared goodies. When you don't respect the regulations, you basically steal others share. Everybody should follow the rules in order the keep the bands usable.
Regulations in the EU
CEPT recommendation which is basically the same.