I don't know whether there's any reliable data about total Bitcoin node that use I2P. https://bitnodes.io/nodes/ currently shows about 22.8K reachable node. But if you search ".i2p" on the search column, it shows 0 node (which should be incorrect).


Yes, it's definitely slower than using clearnet. Just like Tor, I2P use multiple hops/nodes to perform communication. https://geti2p.net/en/about/performance explain about it in detail.