A 10x difference for the same function

% proc t1 {} {variable data; binary encode hex $data}
% proc t2 {} {variable data; binary encode hex -state data ""}
% set data [binary format @102400] ; string length [t1]
204800
% set data [binary format @102400] ; string length [t2]
204800
% set data [binary format @102400] ; time {t1} 10
1755.9 microseconds per iteration
% set data [binary format @102400] ; time {t2} 10
218.6 microseconds per iteration

The C function accesses these Tcl_Obj's using:
    data = input = Tcl_GetByteArrayFromObj(objv[objc-1], &count);
    if (stateObj) {
	data = state = Tcl_GetByteArrayFromObj(stateObj, &state_count);
	count += state_count;
    }
and then I move the 'data' pointer along the state data first then the input data. So the C code is identical for each calling method.