From the computer's point of view, multiple USB devices on a hub represent a single USB device and one interrupt request channel. The bandwidth is divided between all the devices, and only one at a time can be transferring data.
Whether that's going to impact your audio and MIDI devices depends on what other devices are sharing the hub, and how much data is being sent by them. And it can be inconsistent. This is why hubs are generally not recommended for devices such as audio interfaces, video cameras and external disk drives that need to be consistently speedy.
There is also the potential for power supply loading, as noted above. Using a powered hub can avoid that problem, although even powered hubs may not deliver enough amperage for multiple high-current devices.