I have a similar issue. I have used FlasCC to create a trivial SWC (literally one function that just returns an int (2)), and whenever I use autocomplete in my project in flash builder 4.7 it takes an eternity to respond. This is for anything at all, not just members of the library. Member functions in the swc might be even slower than a general autocomplete, but the main problem is that everything is really slow. I think once the autocomplete cannot be something from an SWC then it returns to being fast.
I haven't noticed the project opening being slow or adding the swc being particularly slow however.
This issue is present when using the sample SWC created exactly as in sample 5 in the flascc folder. Having created a similarly simple SWC in Flash Builder 4.7 directly there is no delay in autocomplete.