First let's be clear. Without foundational knowledge of the concepts, getting a good or great score on the GMAT is impossible.
But if you've been studying for a while and you've got good knowledge of the core concepts across Verbal and Quant then what's your best bet for getting the last 20, 30 or even 50 points you need to hit your target?
It's focusing on your in-test process. There a couple great things about focusing on process (how you approach each stage of solving a verbal or quant question):
1) A single process improvement can make a difference on 10 or 20 questions in a GMAT section (quant or verbal) instead of just 1 or 2
2) Process improvements are more about realization and internalization -- so if you're committed and you have a strong credible coach, it can happen in a few days or a week rather than taking months.
The key is those two things: commitment and credibility. The commitment comes from you but can be helped a long by a strong performance coach who has the experience in creating belief and motivation when you're stalled at a particular score level. The credibility comes from the experience of working with 100s or 1000s of test takers -- and having a method that tested and developed through that experience.
To summarize -- fast score jump? ---> it's all about improving your in-test process across questions -- not more practice tests and practice problems.