Of course wallets dont matter this much. But payment gateway definitely does. I mean, this is huge thing for services/businesses/etc. Now exchanges, shops and any other projects could easily accept Bytecoin and other CryptoNote coins. This does matter.
I just havent got it how exactly this would work? Something like CoinPayments or what? How quickly can I implement this on my website, for example?
As it's said in the OP, a new payment gateway will be
capable of receiving and sending thousands Bytecoin transactions simultaneously
Why not make the same feature for GUI wallet?
Why do we need that? Wallet is a wallet, just a GUI. I highly doubt that you will send thousands of transactions in a minute using GUI wallet.
There are two issues that your question raises. As of version 1.0.4 there will be three different wallet implementations for Bytecoin: simplewallet (largely re-written), GUI wallet, and payment gateway. All three are powered with the same IWallet and INode interfaces, so their network performance will be exactly the same.
The difference is in REST adaptor that is used by 3rd party services to process transactions (e.g. check deposits, pay out withdrawals). Currently simplewallet can process up to 200 simultaneous transactions, while payment gateway will be capable of processing up to 10,000 txs at the same time. We expect payment gateway to become the primary wallet for businesses, as it should be much more convenient than simplewallet in this case. Creating such a sophisticated REST adaptor is not a trivial task, and we do not see any need in the same functionality for simplewallet.