Long shot, but if you suspect it's lack of processing power it's easy to try a 3.3V 16MHz Pro Mini.
MaAh
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    RE: [SOLVED] 3.3V 8MHz pro mini as GW, should it work?posted in Troubleshooting
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    RE: 5v vs. 3.3v Pro Mini battery powered nodesposted in General Discussion
A video here https://www.youtube.com/watch?v=kEkD4SLMX8Y on some different ways to power the Pro Mini