The network adjusts the difficulty so that roughly 6 blocks are created network-wide every hour. Each block can contain lots of transactions.
indicates that the difficulty of the POW is increased to halve the block production rate every 4 years. Since this asymptotically approaches 0, it seems problematic. If the rate was a constant 6 blocks/hour then it wouldn't be an issue, but the FAQ suggests otherwise.
If the network grows very large and that makes it difficult to get your transaction into a block for free, you can make it happen faster by paying a small transaction fee to the node that puts your transaction into the block chain. Bitcoin supports this functionality.
This is true. However, if the POW problem is hard enough that even paying transactions have to queue indefinitely for inclusion in a block then they're no better off than those transactions that are included for free in the block (and have to queue for inclusion regardless).