VtArray<float> value;

// This will be a very very very slow loop.
// The non-const array accessor constantly checks to see if the
// data is not-unique (i.e. is being used in more than one place).
// If it is not unique, then the data will be copied.
for(size_t i = 0; i < 100; ++i)
{
  value[i] = float(i);
}

// However, if you used range-based for loops, then that check is performed once
// within the call to begin (rather than each call to []), and the loop body
// will be vectorised by the compiler.
size_t i = 0;
for(auto& f : value)
{
  f = float(i++);
}

// If you need to use the array brackets, then prefer taking a temp copy of
// data() and using that instead. Again, this should vectorise.
float* const ptemp = value.data();
for(size_t i = 0; i < 100; ++i)
{
  ptemp[i] = float(i);
}

// Where possible use const& (Which doesn't perform the unique check at all)
const VtArray<float>& cvalue = value;

// or make use of cdata() rather than data()
auto ptr = cvalue.cdata();