I suspect the transactions would be delayed as the network grows. But really, will it be any slower than a traditional bank wire?
I think not.
If you need a speedy payment and don't really care about privacy just send an IP to IP payment. They are "instant". You'd still want to wait a few seconds to a minute for enough nodes to see the new IP to IP payment to be sure those coins weren't previously promised to someone else.
The client is supposed to only create 15 connections outbound and allow 15 connections inbound. However, I have seen more connected clients on my nodes. The connection code seems to be very lenient.
Essentially (in my mind at least) I imagine stacks and stacks of 30 connection "rings" that are interconnected to other rings. A payment that happens on a node in the same "ring" you are in is fairly instant but to propagate the data to other rings may take some time. A million nodes (let's just say) would be 33,333 rings that are all interconnected in a random pattern. Is my vision correct or did I just inhale too many paint fumes today? ha!
That's my 2c.. err 2 Bitcoins.