Burn speed too slow

I have a sony drive with a 16x read speed and a sony DVD-RW drive with a write speed of 4x. The firmware is up to date, but when I copy another DVD, although the it says that it’s recording at 4.0x, the write speed actually only reaches as high as 1.9x. I have copied lots of DVD’s in the past few days and they all copied at 4x, but now the speed has dropped. I restarted the computer as I thought that may help, but it didn’t. I changed the properties in the device manager for the burner from PIO mode to DMA if available and still no joy.