I use nRF5_SDK_15.2.0_9412b96\examples\dfu\secure_bootloader\pca10040_ble bootloader. I upload my app to my board using PCA10031 and nRF Connect v2.6.2 on Win10 and I can achieve max. 1.3 kB/s speed.
Is that maximum achievable or can it be improved?
As stated in the post Mike linked to, there are no hidden tricks for making the DFU go a lot faster. You can try decreasing the connection interval to make it faster, as well as NRF_FSTORAGE_SD_MAAX_WRITE_SIZE.