The CPU in the Pi 1 is very slow so sending a file encrypted with something like SCP will be somewhat slow. Is it any faster if you install the FTP server and upload it using that ? (EG an unencrypted transfer)
Also are you trying to save the file to the SD card ? Some SD cards have really poor write performance - 2.5MB/sec would be typical write performance of some of the slower cards. It may start out faster while it is being buffered but once the buffer fills it will slow down.
The network performance of the Pi 1 is definitely CPU bound - even on Ethernet it can only just manage about 95Mbps when the CPU is otherwise completely idle. If the CPU is also busy doing something else such as running Kodi, network performance plummets to only half that at most.
The Pi 2 has a much faster CPU and consequently can achieve much better network performance when the system is under load, and the network speeds don’t really fall much if at all.