Another technologically interesting very low energy coin is NANO but it has his own challenges because with the feeless transactions the spamming costs almost nothing.

But still at one point it confirmed more transactions than BTC + ETH + LTC combined without tx fees and sub-second confirmation time:

https://www.reddit.com/r/nanocurrency/comments/lxbhh5/nano_c...

The spam is becoming a too big problem, but they're trying to mitigate it in the new versions:

https://www.reddit.com/r/nanocurrency/comments/namwzf/v22_hi...