Need help faster way to read byte from serial

[WhatMonday]

Hi All,
I am at a beginner level. I want to read all byte from serial until &H29. I have simple code below but it take over 6 seconds.
Could you help to make it faster? or share better method. ( I try same data with commercial software it less than 1 sec )

    Function GetBytes() As String
        Dim serialbyte(350000) As Byte 'For example set maximum byte is 350000
        Dim count As Integer = 0
        Dim i As Integer = 0
        Dim rcb As Byte
        While rcb <> &H29
            rcb = SerialPort2.ReadByte
            serialbyte(i) = rcb
            i += 1
        End While
        Dim str = "Nothing"
        Return str
    End Function