arrays of structs - performance

I am evaluating swift for a certain performance sensitive app and I am hitting a performance wall ( pretty terryfing one at that ) which I am trying to understand ..

I have a struct defined as:

struct DataBlock
{
    var counter:UInt=0
    var values:[UInt64]=[]
}

Then I crate an array of these structs :

var dataArray:[DataBlock]
dataArray=Array(count: aSize, repeatedValue: DataBlock())

My understanding here is that this should behave more/less like C++ equivalent with a continous array of DataBlock structs - in terms of repeatedly accessing content of that array it should be cache friendly.

Unfortunately this ends up being extremely slow ( 10 times slower than an equivalent Java code and 40 times lower than C++ ) - all of that compiled/tested with -OFast.

When profiling I see basically release/retain taking most of execution time which, I think , means I don't rally understand structs in Swfit - looks like there is still some boxing/unboxing going on big time.

For instance , if I extract the array size into a variable and index on that , the code becomes twice as fast

            let valuesArraySize=dataArray[i].values.count          
            for var j = 0; j < valuesArraySize; ++j

This, on the other hand, is twice as slow:

            for var j = 0; j < dataArray[i].values.count; ++j
            {

So obviously, this points to data being retained/released on every access within the array which is really odd given that I am using structs ...

Any ideas what I am doing wrong ?