I suspect it means something like "according to calculations using a noticeably approximate model of reality". That's what people tend to mean, anyway. "Theoretically" is often code for "My model does not correspond to reality".
It annoys me, actually, that "theory" has this negative connotation of noncorrespondance.
How can something be theoretically slower? What's the difference between theoretically slower and practically slower?
Only practical implementations have a speed that you can talk about.
I don't know what you mean by 'theoretically' as an adverb there.