Speed is not a function of capacity in this particular context. A car with 1 seat driving 500 m/h is faster, don't you think, than a car with 4 seats that drives 50 m/h.
I mean we can argue semantics, but if you want to send $100 from the US to the Philippines, bitcoin is generally a lot faster than your bank, even when you have to buy bitcoin and sell it in the Philippines because of international transactions taking much longer than local ones.
To make a long story short, thousands of transactions per second (Visa averages about 2k) is certainly possible. I'd be more worried about bitcoin adoption (from an interest/tech/growth point of view) as a limit than technical scalability issues as a limit, especially with things like sidechains. (Look at Blockstream's Sidechains for example as a solution, just got a $21m investment.)
It's generally held that it can scale, whether it can grow and become popular and well used remains to be seen (I think it's likely but it's far from certain).
Banks don't transfer money to each other through Visa, they use SWIFT. Transactions usually take 4 business days.
The block size limit is in the process of being changed, but there isn't a rush since blocks are not being filled. If they do get filled, transaction costs will go up and tiny (0.01 USD) micro-transactions won't be viable until the block limit is increased. So the only thing that might happen is transactions of a few cents are temporarily not viable (the transaction cost is larger than the amount sent).
If your wires take you 4 days you need to switch banks. I make regular international transfers and unless I miss business hours they are always in the receiving account the next morning.
