If store and load instructions for the same data exists close enough in the pipeline, the CPU can skip storing it to cache and just forward it directly to the load (store forwarding). This can save several CPU cycles. There are different issues with alignment, but basically the store forwarding may not work if the data isn't aligned to the right memory address. It might end up being aligned without using the attribute, but it might not, and I think that's why you see different performance with the same code. The attribute guarantees that it will be aligned every time.
As for VTune, you can go to the Bottom-up tab and see the list of functions along with the TMA metrics for each function. If you find the one with high Loads Blocked from Store Forwarding, you can double-click on the function and bring up the source view. Depending on certain configurations, you can see the line of code affected by the stall, and then see whether the variable is aligned or a smaller size than the read.
