Question is asking about the speedup in a theoretical system where part of it has some enhancement. Why are you taking into account cache misses and other factors ?
If half of the road from Delhi to Agra is a new highway that lets you drive at twice the speed limit - and somebody asks you what is the time improvement in driving from Delhi to Agra, you won't respond by taking into account traffic jams, cows on the road, etc right ? Same thing here. Question is asking speedup of system with enhancement over system without enhancement. How efficient that enhancement is - is beyond the scope of the question.