Probably a good idea to first talk about what kind of time quantity vs row quantity we're talking about, as well as the hardware info, and in particular, the speed...

Are both encryption key files 100% identical ?

Interesting question, but I have to ask what the meaning of "number of items available" is, given any period of time longer than 1 day.   During your period of 8/3...

Just a thought, but have you tried to directly decrypt in the SELECT of the message data, as opposed to putting the data in a variable first?
Also, are you...

Okay, but if your tool isn't doing the job, why use it?   SSIS can accomplish the task effectively.   Alternatively, perhaps you can look at the Talend documentation and see if...