Serial port communication very slow

Public Shared Function ReceiveData() As String
            Dim RXCnt As Integer = 0
            Dim RXByte As Byte = 0
            Dim reply As String = String.Empty
            Dim STX As Byte() = {&H2, &HD}
            Dim hexwrite As String = String.Empty
            Dim ack As Double = 0
            Dim errorCode As String = Nothing
            Dim strText As String = Nothing
            Dim RXArray As Char() = New Char(2047) {}
            Dim errorDescription = String.Empty

            Dim LookUpTable As String = "0123456789ABCDEF"

            Do
                '----- Start of communication protocol handling ---------------------------------------------

                RXCnt = 0
                Do
                    RXByte = COMPort.ReadByte

                    RXArray(RXCnt) = LookUpTable(RXByte >> 4) ' Convert each byte to two hexadecimal characters

                    RXCnt = RXCnt + 1
                    RXArray(RXCnt) = LookUpTable(RXByte And 15)
                    RXCnt = RXCnt + 1

                Loop While Not ((COMPort.BytesToRead = 0))

                '----- End of communication protocol handling -------------------------------------------------------------

                recievesend = False

                reply = RXArray(0).ToString & RXArray(1).ToString

                If reply = "06" Then
                    If readwrite = True Then

                        Try
                            COMPort.Write(STX, 0, STX.Length)
                        Catch ex As TimeoutException
                            errorCodeText = String.Concat(errorCodeText, ex.Message)
                        Catch ex As InvalidOperationException
                            errorCodeText = String.Concat(errorCodeText, ex.Message)
                        Catch ex As UnauthorizedAccessException
                            errorCodeText = String.Concat(errorCodeText, ex.Message)
                        End Try

                        recievesend = True
                        hexwrite = "020D"
                    ElseIf readwrite = False Then
                        If ack < 1 Then
                            ack = ack + 1

                            Try
                                COMPort.Write(writebytes, 0, writebytes.Length)
                            Catch ex As TimeoutException
                                errorCodeText = String.Concat(errorCodeText, ex.Message)
                            Catch ex As InvalidOperationException
                                errorCodeText = String.Concat(errorCodeText, ex.Message)
                            Catch ex As UnauthorizedAccessException
                                errorCodeText = String.Concat(errorCodeText, ex.Message)
                            End Try

                            recievesend = True
                            hexstring = StringToHex(writedata)
                        Else
                            ack = 0
                        End If
                    End If
                ElseIf reply = "15" Then
                    errorCode = RXArray(3).ToString()
                End If
                Thread.Sleep(300) 'sleep between incoming bytes
            Loop While Not ((COMPort.BytesToRead = 0))

		strText = New String(RXArray, 0, RXCnt) & vbCrLf

            Return DecodeHexString(strText)

        End Function