Thanks for the link, I experimented with the settings provided and I maybe saw a small increase (~0.2KiB/s) but not major boost.
My client test devices have been:
- a 4.0 Bluetooth dongle and Ubuntu using the newtmgr (go version) -> 5KiB/s
- Android Pixel 3 XL with Bluetooth 5.0 and nRF Connect app -> 5KiB/s
- Cheap Android 8.1.0 tablet with Bluetooth 4.2 and nRF Connect app -> 1.25KiB/s
I was talking with Vinayak and he was saying the DFU flash writes will cause the CPU to halt (and BLE radio will not be in use) so this will likely reduce throughput somewhat.
I'm trying to send him a sniffer capture of the DFU transfer but I don't have a 2Mbit PHY sniffer as yet.
My results also correlate with this github issue: https://github.com/zephyrproject-rtos/zephyr/issues/19244#issuecomment-534424966
Also I found this link with similar results for the Nordic SDK so it might be the flash writing issue:
If I get a chance I'll try DFU with an external flash to compare.