Currently I have a laptop that has Express Card slot but no USB 3.0 slot and I notice that there are a lot of Express Card to USB3.0 converter for sell. My question is about the speed. Can the speed of an Express card to USB 3.0 converter reach close to the native USB3.0 slots on the motherboard? ie. Can the Express Card solution really release the potential of the speed improvement of USB 3.0? By speed I mean the average write speed.
USB 3.0 in theory has twice the throughput of most current ExpressCard implementations, so the answer from a technical standpoint is 'No, it cannot'. ExpressCard 2.0 is fast enough to do it, but I don't think that's really hit the market in any significant way (and you'd need an adapter that supported it as well to take advantage of the higher speed).
However, there are few (if any!) USB 3.0 devices on the market that are close to saturating the speed of the USB 3.0 bus as it is, so in practical terms you are unlikely to notice a significant difference. (This assumes that the ExpressCard adapter you choose doesn't add a significant amount of overhead, which I personally feel is probably a relatively safe assumption to make.)