Reusing a one-time pad?
Use AES-128, the instruction set in most CPU's (AES-NI) speeds up the encryption and does not put to much load on your CPU. I would use CBC but there might be better mode operations for encrypting files. Also don't forget to use a MAC.

Using a one time pad (OTP) is nice but what you're doing is not an OTP it's more a Vigenère cipher. If you were to use OTP the key should be as long as the file which requires you to store a (approximately) 200MB key. 

Thanks @CodesInChaos I missed the "I have an embedded device", but than it's not a CPU it's a MCU (most likely) even than, AES is designed to be fast on any platform so i would still give it a try, just see how much it takes i think you'll be surprised as to it's efficiency.

Take a look at the list of validated AES implementations, see if there's something which resembles your embedded device.