The speed of execution limitation is not because of 0x, but because of how fast a tx can be registered on the block chain. It can't compete (they say for now) at settlement speed with a high performance centralized exchange.

But you have to trust those centralized exchanges with your funds. While with 0x you don't have to. One could say centralized exchanges can't compete on trustlesness with decentralized ones.

I completely agree. Until the blockchain can confirm transactions at lower latency I don't think the 0x protocol can provide a stand in replacement for a centralized exchange. I'm currently working on a project that goes about settlement a different way so trades can be confirmed instantly given a little trust. If you're curious, here's some details on how it works https://merklex.io/blog/decentralized-clearing-network/