We test our application code (size~ 30KB) together with original hello-sensor sample code (size~8KB);
we find ota time of T(hello-sensor) is Ta and our application code is Tapp where Tapp > Ta too much.
We observe the reason why Tapp is bigger than Ta is owing to code size during OTA wireless transfer and
synchronous program/erase speed dependent of code size. Besides that, any other bug/reason we can
improve the OTA speed from handset side?
Besides, OTA speed seems to have something to do with OS system on handset like android phone/iOS phone;
how to prevent OTA speed change if user upgrades a higher version OS on phone?
If you want to prevent the changes in speed with respect to OS, then you will need to modify the App source code.It is App specific feature.