Taking the easy one first…
As far as I’m aware, the reasons aren’t documented. You need to know the history.
It wouldn’t be 50 times in 10 seconds. If you were using all four c.t. inputs, it would be 12–13 times in 10 seconds, because the channels are checked sequentially. And the limitation is emoncms.org, as I mentioned above. emoncms.org preceded emoncms on the Raspberry Pi by many years. Although some users had access to a “full-size” server, there was no standard way to have local storage before the emonPi was introduced.
That’s called emonLibCM, and it’s been the standard sketch for the emonTx since mid-October, 2019. You can get the latest release and documentation at EmonLibCM - Version 2.2.2
The Atmel '328P spends roughly half its time processing the interrupts from the ADC, which probably doesn’t - unless your processor is significantly faster - leave a lot of time to do other things. For example, it can use the RFM69CW radio as a transmitter because that has an internal buffer that can be filled as it wishes, it cannot use the RFM12B because that doesn’t have an internal buffer and the timing of each byte sent to the transmitter is critical. So the “classic” JeeLib library can’t be used with emonLibCM.