Does a USB hub affect performance?

Yes. All devices connected through a USB hub share the bandwidth available to that hub. Not specifically at 50% each though. You've got 480 Mbit/s to work with per USB 2.0 hub at the computer. What do I mean by that? Well, your computer has USB hubs built into it. Yes, not every port is an individual entity. In most cases, when you see two USB ports stacked one on top of the other, they are on a hub together internally.

This also applies with two ports side by side on laptops. So, don't think you can just plug two hubs into USB ports that are side-by-side, and have LOTS of USB ports to plug high data transfer rate devices into.

What you REALLY need to do is look at the expected data transfer rates of the devices you intend to connect. I'd expect that the MIDI drums will be considered a low transfer rate, while the keyboard will either be a low or medium transfer rate. This would be compared to something like a USB sound card... which you would not want to share a hub with anything else.

With a powered hub, each device will get the power it needs, while with an unpowered hub, all devices share whatever power the host USB port can put out. So, there is that to consider as well.

You most likely already know, but for latency issues, ASIO4ALL drivers will cure most if not all potential issues. Just putting that out there.

Actually, I'm surprised the first anwer is accepted and upvoted without any facts whatsoever to support the statement, as it's most probably a wrong one. Both MIDI drums and MIDI keyboard are almost certainly low-speed devices, so they will consume less than 1% of bandwith from a high-speed hub at most (2*1 Mbit/s / 480 Mbit/s * 100% = 0.4%).

Indeed, the presence of the hub will introduce a latency, which is of order of tens of microseconds for low-speed hubs or hundreds of nanoseconds for high-speed hubs. In the latter case, this latency will vanish once you add the latency introduced by the MIDI software.

Also, USB protocol supports transfers priorities (see Interrupt transfers), which will allow MIDI devices to coexist even with a hard drive or a scanner on the same bus without much effect on their transfer speed or latency. However, I won't make any statements since I'm not familiar with MIDI devices in particular.