It stands for "kibibyte per second", i.e. 1024 bytes per seconds:
https://en.wikipedia.org/wiki/Data-rate_units#Conversion_table
https://winscp.net/eng/docs/scriptcommand_synchronize

Hello,

I am using WinSCP 4.3.6.1655 to do automated file transfers via FTP between Windows hosts. I am using the synchronize script command with the speed option to try to throttle the transfers. The speed option says to specify a value in kibps (-speed=<kibps>). What exactly does kibps stand for? Kilobits, kilobytes, kibibits, kibibytes?

Thank you,

Patrick