IIRC, limit to list does not do what you describe (filter as you type). That can occur if you have Auto Expand set to yes. It is also often done with code. You seem to be saying that the code is not viewable, so is the db an .accde file? If so, the original version that the accde is based on should be able to provide it. If it's accdb or mdb then the code should be viewable. So if your combo filters via code, I don't see how we can help much if we can't see it. If you are able to post a db copy, see How to Attach Files at the top of this page.
EDIT - since you didn't mention it, I presume that performance of other applications is OK (like Excel)?
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.