Reserving more spaces to make it a full cache line probably won’t make a performance difference.

More importantly, you should align this structure to a cache line. If the structure spans multiple cache lines, it may waste cache memory. (Though, whether this leads to significant performance degradation is questionable.) Additionally, depending on the processor architecture, spanning multiple cache lines could lead to increased cache misses or excessive memory bandwidth usage during writebacks.

In any case, if your performance requirements are extremely strict, it might be worth testing and comparing to see if there is any actual performance difference.