Why is this the case? When I2P finds its peers and connects to your wanted hidden service then traffic is exchanged almost instantly. More nodes in the I2P network = faster network.
Test it out with I2P BitTorrent client. You will quickly connect to peers and download at ~100 KB/s. 100 KB/s is the speed you can achieve with current I2P nodes as there aren't that many. Imagine what would happen to the I2P network if 2000 fast-nodes would go under it and start routing traffic? Not only it would improve anonymity but also decrease latency and improve speed.
Tor should be used if UDP support for masternodes is not required because Tor does not support UDP while I2P does. I think masternodes should go under Tor or I2P for anonymity and ddos protection reasons.
I might not know anything about cryptocurrency but when it comes to networking, malware and botnets I can fairly say that I am quite the expert. My hobby