DFU file upload speed through BT SMP is painstakingly slow especially for Matter firmware with MCUboot which are about 800 kB.
I just simply compiled and provisioned Matter lamp example on nRF52840 DK and without modifications the upload speed was around 1.5 kB/s from iPhone 14.
After the following modifications (suggested here):
BT_L2CAP_TX_MTU=498
BT_BUF_ACL_TX_SIZE=251
BT_BUF_ACL_RX_SIZE=502
BT_CTLR_DATA_LENGTH_MAX=251
it became 3.7 kB/s. It's better but still much slower that I would expect. Is it possible to improve it a bit more?