When I started preparing for GMAT, I was struggling with my timings and my thought process was clear only over a limited range of topics in quant. GMAT Club tests helped me with both. The best part about GMAT Club tests is that they provide lots of variations in any topic they frame questions from. Although I found the actual test a little easy, GMAT Club tests are good indicators of how well one understands the concepts. The questions here usually require analysis of multiple concepts at once. With sufficient practice, it becomes easy to gauge at the outset, what concept the question is going to test. I have not come across a better set of tests where one can work towards improving timing. In my opinion it's best if all 26 tests are given and a track of performance analysis provided after each test should be studied. I would highly recommend GMAT cub tests to improve Quant score.