I maybe installing an IDS that has a maximum throughput of 150 Mbps. Instead of putting it inline on my main gigabit port to the rest of the network, I would like to just mirror that port and install the IDS on that. The question I have is will I still see performance hit on the port that I am mirroring when the mirrored port is connected to such a slow device?
No. I suppose that your IDS could "drop" mirrored traffic in excess of its capacity, but it would not affect anything upstream.
One thing to consider is that your switch may allow only one mirroring configuration, to a single port; is there a chance that you would ever need to mirror anything else on the same switch?