If you're using it to browse webpages on the clearnet then yes, it's slower, because that's not the use-case for i2p. I've seen speeds around 1MBit/s which isn't lightning fast, but also not terrible for an anonymous network.
That's because you're using it for a purpose it wasn't intended. I2P isn't designed to be used to browse the regular internet, for that it's better to use TOR. However for anonymous torrenting or accessing i2p-sites, it's quite fast imho.
Very easy to understand analogy. Thanks for that. Makes it very clear on how it works. Doesn't sound like a too small hose should cause problems then?