The burning speed of SL and DL is the same. 1x = SL is the same as 1x DL.
The only difference is the amount of data that you can burn.
SL = 4.4Gb DL = 8.5Gb.
So if you would burn a full SL disc and a full DL disc the DL disc would be a little bit less than 2x the recording time of the SL disc.