doc: add info to i2p.md about IBD time and multiple networks

Github-Pull: bitcoin#22589
Rebased-From: d2dffd5

doc/i2p.md

@@ -47,13 +47,21 @@ information in the debug log about your I2P configuration and connections. Run
 `bitcoin-cli help logging` for more information.
 
 It is possible to restrict outgoing connections in the usual way with
-`onlynet=i2p`. I2P support was added to Bitcoin Core in version 22.0 (mid 2021)
+`onlynet=i2p`. I2P support was added to Bitcoin Core in version 22.0 (mid-2021)
 and there may be fewer I2P peers than Tor or IP ones. Therefore, using
 `onlynet=i2p` alone (without other `onlynet=`) may make a node more susceptible
 to [Sybil attacks](https://en.bitcoin.it/wiki/Weaknesses#Sybil_attack). Use
 `bitcoin-cli -addrinfo` to see the number of I2P addresses known to your node.
 
-## I2P related information in Bitcoin Core
+Another consideration with `onlynet=i2p` is that the initial blocks download
+phase when syncing up a new node can be very slow. This phase can be sped up by
+using other networks, for instance `onlynet=onion`, at the same time.
+
+In general, a node can be run with both onion and I2P hidden services (or
+any/all of IPv4/IPv6/onion/I2P), which can provide a potential fallback if one
+of the networks has issues.
+
+## I2P-related information in Bitcoin Core
 
 There are several ways to see your I2P address in Bitcoin Core:
 - in the debug log (grep for `AddLocal`, the I2P address ends in `.b32.i2p`)