That is what Intel shows as max Turbo Speed: https://ark.intel.com/products/88967/Intel-Core-i7-6700HQ-Processor-6M-Cache-up-to-3_50-GHz, the base speed is 2.6
Turbo Boost is a dynamic feature. There is no set-in-stone speed which the Core i5 or i7 processor will reach when in Turbo Boost. Turbo Boost operates in 133Mhz increments and will scale up until it either reaches the maximum Turbo Boost allowed (which is determined by the model of processor) or the processor comes close to its maximum TDP. For example, the Core i5 750 has a base clock speed of 2.66GHz but has a maximum Turbo Boost speed of 3.2GHz.
However, Intel still advertises these processors by their base clock speed. This is because Intel does not guarantee that a processor will ever hit its maximum Turbo Boost speed. I have yet to hear of an Intel processor which can’t hit its maximum Turbo Boost speed, but hitting the maximum Turbo Boost is dependent on workload – it won’t happen all of the time.
http://www.makeuseof.com/tag/forced-induction-intel-turbo-boost-works-technology-explained/
Try doing a high demand single processor test and see if it gets to 3.5, it only works on 1 core, it won't do all of them.
Mine only boosts to 3.9 but its core is 3.5, yours has a much larger range.
It depends on temperatures more than anything, if CPU is running hot it won't run turbo boost, I don't see it very often but PC seems fast enough now without needing CPU running full speed, its ticking along at .88ghz and it doesn't feel slow at all. If I need it do anything it will speed up again. I could overclock but i need a better CPU fan before i do that, and once again, PC doesn't feel slow now so I leave that a few years.