Slim Thug says women who claim to have been abused need to speak up when it's actually happening -- which is why he thinks Cassie's lawsuit against Diddy came too little, too late.

The rapper aired out his thoughts online with a very honest video, in which he spoke on the Cassie/Diddy situation -- which is now resolved after the two of them reached a settlement in record time.

Still, ST spoke his piece ... and he says he simply doesn't support people coming after their alleged abusers years after the fact -- and not only did he mention Diddy and Cassie in making this point, but he also name-dropped Bill Cosby. As he says, he ain't with all that.

Slim also dished his opinion on why he believes Cassie even filed her suit with bombshell claims -- and, according to him, it may have to do with her living a different lifestyle now.

Bottom line ... Slim Thug's saying he thinks this is all about money, and has a hunch Cassie was simply out for cash in this whole situation. Aubrey O'Day appeared to echo that as well.

In any case ... Slim's bigger point here is, the post-mortem undressing of old (alleged) incidents not only needs a statute of limitations legally -- but, perhaps, socially too.