Hi and welcome to the forum
Burning speed is related to the disc. Some discs cannot be written at 24x, so the firmware cut burning speed at the correct value.
Burning software has no role in this because is firmware that control this
What discs are you using? Did you try to update firmware?
Moreover, a burner is not able to write at full speed even if the disc can support a high speed if the drive doesn't receive data fast enough. This can happen if hard disk is too full and/or too fragmented and/or if it must provide data also to other applications. For example, if you are running a software like antivirus scan or torrent/e-mule and you are burning at the same time, the drive cannot receive data fast enough to burn at full speed.