no performance improvement with std::thread - c++

Question

I am working on a audio "real time" application and I would like to imrpove the performance of it. I actually already posted a topic but this is about std::thread specificly.
The audio processing ist mostly done by two seperate objects ( leftProcessor track and rightProcessor). Since these objects don't rely on each other, using two threads to process them should realy improve the performance with multi core CPUs. However, I currently get the opposite result.
Before I activated the compiler performance optimization (O2), using two threads got me about 50% more performance, but after I switched the optimization on, I got ~10-20% less performance, while the performance got drasticly better for both versions.
I measure the performance by taking the time at two points within the function and bruteforce printing the result to the screen so there could be some problems with this. ;)
My guess would be that creating a std::thread would take more time than I actually gain from running the processing on the second thread.
In this case, is it possible to improve the performance by using the same "thread" for every function call and just passing the thread the new arguments? I don't realy know if this is possible.
The function currently takes about 0.0005ms to 0.002ms to process.
Here is the code:
void AudioController::processAudio(int frameCount, float *output) {
#ifdef AUDIOCONTROLLER_MULTITHREADING
// CALCULATE RIGHT TRACK
std::thread rightProcessorThread;
if(rightLoaded) {
rightProcessorThread = std::thread(&AudioProcessor::tick, //function
rightProcessor, //object
rightFrameBuffer, //arg1
frameCount); //arg2
} else {
for(int i = 0; i < frameCount; i++) {
rightFrameBuffer[i].leftSample = 0.0f;
rightFrameBuffer[i].rightSample = 0.0f;
}
}
#else
// CALCULATE RIGHT TRACK
if(rightLoaded) {
rightProcessor->tick(rightFrameBuffer, frameCount);
} else {
for(int i = 0; i < frameCount; i++) {
rightFrameBuffer[i].leftSample = 0.0f;
rightFrameBuffer[i].rightSample = 0.0f;
}
}
#endif
// CALCULATE LEFT TRACK
Frame * leftFrameBuffer = (Frame*) output;
if(leftLoaded) {
leftProcessor->tick(leftFrameBuffer, frameCount);
} else {
for(int i = 0; i < frameCount; i++) {
leftFrameBuffer[i].leftSample = 0.0f;
leftFrameBuffer[i].rightSample = 0.0f;
}
}
#ifdef AUDIOCONTROLLER_MULTITHREADING
// CALCULATE RIGHT TRACK
if(rightLoaded) {
rightProcessorThread.join();
}
#endif
// MIX
for(int i = 0; i < frameCount; i++ ) {
leftFrameBuffer[i] = volume * (leftRightMix * leftFrameBuffer[i] + (1.0 - leftRightMix) * rightFrameBuffer[i]);
}
}