In the past I have had occasions when my users opened more than one form at a time. This arose from a design in which each form showed purity of purpose (for the most part.) That is, if you opened the Computers form you were updating information about the computers on our site. If you opened the Projects form you were updating project information, and so on. I had Users, Computers, Projects, Teams, Tasks, and a few other items having to do with workflow and accountability issues.
The question is, what happens in the workflow to cause people to WANT/NEED to open multiple forms at once? When the concern is that someone wants to open multiple forms, what drives that approach? Are they overloaded and trying to do too much at once due to unrealistic expectation or is the workflow complex enough that you need to open multiple forms to get something through your work process? The question of performance should be backed out in scope to include the human factors relating to performance. What drives people to appear to want to do multiple things at once?
But it goes further than that. Every time you try to do multiple things at a time through having multiple forms open, you run into the problem that each form has its own context, which is normally independent of everything else. The USERS have that many MENTAL contexts, one for each form that is open, so they have to shift attention from one to the next.
So... let's say (using my old project as the basis) that you wanted to update membership of a project team, so you use the Teams form. Voila, person added! But now you have a task you want to assign to that person because he is now a member of the team. So you go to the Tasks form, WHICH WAS ALREADY OPEN. For the next 30 seconds or so, you might be unable make the assignment because the form's context had already looked up team members eligible for the action and the auto-refresh is 30 seconds. You want to assign the user but the combo-box of eligible users has ALREADY QUERIED the list of team members. You need to refresh the form that was already open to force it to recognize the new member.
Granted, my case is hypothetical because I had actually solved that problem in other ways, but the point is that EVERY OPEN FORM has its own context that was updated at the most recent navigation event. Having too many forms open at once runs the risk of destructive self-interference (a.k.a. "the left hand doesn't know what the right hand is doing.") So to the extent that there may be an OPERATIONAL-level issue, WHY do you need 3-5 forms open at once? Would a form change to incorporate a different viewpoint reduce the NEED for that many forms being active?
All too often we focus on machine factors in performance, but we have to remember that Access forms, being human-interactive, also depend on "think time" as a link in the performance chain. The question about performance has been answered in various ways. Open, idle forms incur no CPU load but will incur memory load and back-stage resource loads. But what about the people using this scenario?