NZXT fan hub problem - fan speed not adjustable/running low

You have not taken into account the limits of fan systems.

The root of the problem is this. Any mobo fan header can accept from its fan ONE speed pulse signal sent back from the fan motor. It can count those pulses to display the speed for you. BUT if you use a Splitter or a Hub to connect more than one fan to that single header, the system must NOT send back the speed signals from all the fans. The mobo pulse counter system could not handle that confusion and would create all kinds of errors. So ALL Splitters and Hubs send back the speed signal from only ONE of their output ports.

Now, it is VERY important that the cooling system for the CPU should send its speed signal back to the CPU_FAN header. That header does extra monitoring of the cooler's speed to be sure to protect the CPU from overheating. The Hub included with the NZXT case has a connection to the CPU_FAN header that picks up the PWM signal it needs to do its job, and sends back to that header the speed only from its white Port #1, which happens to have 4 pins. You have followed instructions well and plugged into that #1 port the Corsair CPU cooler system so its speed is being sent to that header. But the Hub can NOT send back to any mobo header the speed signals from any other of its ports. The speed signals of ALL of those case ventilation fans is simply ignored - not connected to anything. You will never see their speeds measured. The only speed you will ever see, no matter which mobo header you plug into, will be the Corsair cooler system. And you REALLY should plug the lead from the Hub into the CPU_FAN header so it can be monitored properly.

Now, you first concern was that the four case fans seem to be running slowly. That may be just what they should do. They should run slowly if you are doing very little work, and speed up later if you start doing a heavier job. If you want to make a small check, try this. You need to have your system cool before starting this, and then arrange to be able to see the speed of one or more of those case fans. Maybe you need to open the case, maybe you can see from the outside. Anyway, what you need to do is watch one or more of those fans during the few seconds just at start-up. Why? Well, the "normal" process for automatic fan control is that right when you start up the fans all will start at full speed. Then in just a few seconds as the POST process completes and the system can get a temperature reading from its sensors, it finds that the temperatures are low and it slows down the fans. So, if your case fans start fast and then slow down in a few seconds, the automatic control system is working properly.