visual c++ Serialport class handling data very slow - multithreading

Question

I'm trying to handle data recieved at about 1Hz from a robot through a serialport (bluetooth connection). I will recieve data with different headers determining what data will be recieved and what the expected length of the message is.
for example:
Header: sensorvalues (0x32) -> expected length 11 incl. header.
First i want to check if the byte is a header and if thats the case extract the expected length (in bytes).
I am using vc++ and the serialport class in the GUI.
The "seirialport->read(buffer, 0, expected length)" is very slow and not very reliable. I found some tips at http://www.sparxeng.com/blog/software/must-use-net-system-io-ports-serialport but the buffer still builds up to the point where I have about 200 bytes in the buffer, which is not very good when i need live sensorvalues displayed on the GUI.
The key parts of my code are:
System::Void MyForm::serialPort1_DataReceived_1(System::Object^ sender, System::IO::Ports::SerialDataReceivedEventArgs^ e) {
if (serialPort1->BytesToRead > 0){
if (write_position == 0){
serialPort1->BaseStream->ReadAsync(data_recieved_buffer, 0, 1);
header = data_recieved_buffer[0];
if (this->InvokeRequired){
myrecievedata_delegate^ d = gcnew myrecievedata_delegate(&myrecievedata);
this->Invoke(d, gcnew array < Object^ > {'h'});
}
else
{
myrecievedata('h');
}
}
else if (this->serialPort1->BytesToRead > expected_length - 1)
{
serialPort1->BaseStream->ReadAsync(data_recieved_buffer, 0, expected_length - 1);
if (this->InvokeRequired){
myrecievedata_delegate^ d = gcnew myrecievedata_delegate(&myrecievedata);
this->Invoke(d, gcnew array < Object^ > {'b'});
}
else
{
myrecievedata('b');
}
}
else{
return;
}
}
and the recieved data is sent to
System::Void MyForm::myrecievedata(char status){
if (status == 'h'){
handleheader(header);
}
else if (status == 'b'){
handlebyte();
}
Is the problem at the serialport_datarecieved event? I can only think of invoke (which I have very little knowledge about) being the problem, still keeping the work in the serialport thread.
If that is the case how would I make sure that the data is handled in a different thread?
Thanks in advance!