Excel Solver slow due to second workbook - vba

Question

I've noticed something new today that will cause me problems in the future regarding Excel's Solver, so I want to get a jump on it.
I'm using VBA to sequentially apply solver moving down a sheet (let's say this sheet belongs to 'workbook 1'). This all works fine and I'm happy with the results of Solver. Yay... My concern is this: When I have a second rather large workbook open (let's call this 'workbook 2'), WHICH HAS ABOSOLUTELY NO LINKS TO MY CURRENT WORKBOOK, Solver takes around 20 times as long to run.
Naturally the most reasonable thing to do is not have 'workbook 2' open when Solver is running. For now that's my solution, however, in the very near future I will need to have both workbooks open simultaneously. So I pose the following question:
Why would solver take longer to run with two books open even though it's not even touching one of them?
In case skeptics are concerned that my code is doing something unusual here it is:
Sub ExampleSeqSolver()
Dim Iter As Long
Dim Time0 As Double
Dim Duration As Double
Application.ScreenUpdating = False
' Solver requires that it is working on the 'active sheet', silly but w/e
Sheets("Sheet1").Activate
' watch optimization efficiency
Time0 = Timer
For Iter = 2 To 13
' my data are sometimes related to adjacent rows so I was considering supplying an initial solution to reduce the number of runs
' Sheets("Sheet1").Range("$AC$" & ITER & ":$AI$" & ITER) = _
' Sheets("Sheet1").Range("$AC$" & ITER - 1 & ":$AI$" & ITER - 1)
If Cells(Iter, 1) <> 0 Then
SolverReset
SolverOptions AssumeNonNeg:=True, Iterations:=100
SolverAdd CellRef:="$AK" & Iter, Relation:=2, FormulaText:="1"
SolverOK SetCell:="$AW$" & Iter, MaxMinVal:="2", ByChange:=Sheets("Sheet1").Range("$AC$" & Iter & ":$AI$" & Iter), Engine:=1
SolverSolve True
End If
Next Iter
' report optimization duration
Duration = Round(Timer - Time0, 2)
MsgBox "Optimization finished in " & Duration & " seconds", vbInformation
Application.ScreenUpdating = True
End Sub

Answer 1

The presence of 'workbook 2' is correlated but not causal. In my case there is one particular workbook that causes the solver slowdown; most others don't. The workbook that causes the slowdown is about 4mb. Much larger workbooks do not cause a slowdown. The one causing the slowdown does not to the best of my knowledge contain either macros or VB. I recommend that this problem be shown to Frontline systems who developed the original solver.