I'm bidding because this is intriguing to me. So, the idea is that the current i/o card accounts for 75% of the workload and we want this workload to be 50% faster. We also want the other 25% of the system workload to be 50% faster, but we have no control over that in this scenario.
That means, I believe, that we need to increase the card pulses by 50%, of course, which takes care of the card workload.
We also need to account for the remainder of the system, to achieve our total 50% increase. That means that we need to add, at least, another 25% more pulses to the card. Therefore, it would be 5kHz(50%) + 2.5kHz(25%) = 7.5 kHz + 10kHz (current card) => 17.5kHz rounded to 18kHz.