On the Pi 2, the ethernet port was really a USB-to-ethernet port plugged into a hidden USB port. So network speeds were limited to USB 2 speeds. I'm guessing the Pi 3 also works like this, instead of having pure onboard ethernet? But I don't know yet.
Given that USB 2.0 is 480Mb/s, and Ethernet is 100Mb/s, there is no bottleneck when using the Pi's Ethernet port. I really wish this myth that "Pi Ethernet is slower than dialup because USB" would just die.
Of course, like everyone else I'd love to see a new version of the Pi with Gigabit Ethernet directly connected to the system bus, and USB 3.0 while we're at it. But that would greatly increase the cost of the device and also require a newer SoC that supports such things.
I can't find any info to say which bus the MMC reader is connected to, but I know if you use a USB drive as well as the built in ethernet port, speed will suffer.
If it's using the same bcm2836 chip, then I think it'll have the same limitations. The markings on the chip in the pictures aren't quite legible enough to me, playing around in a photo editor, but it's marked the same way that the Pi2's chip is, from what I could see.