Loop Referencing Sheet Gets Progressively Slower

I have a simple function that pulls in a single cell value, one per row, stores it in a property of a custom type, and then does a little more processing based on the value.

The custom type is GR, the property is CGID.

Code:
With Worksheets("Sheet1")
GR.CGID = UCase$(Trim$(.Range("CGID")(rw).Text))
End With

It runs fine with no noticeable performance changes up to around 10,000 records, but beyond that, it gets progressively slower and slower and slower until, at about the 20,000 record mark, it's processing at about 1/10 the speed it started at! (100 recs/sec instead of 1000+)

For testing purposes, all the cell values are the same, and are a small string value.

I know that hitting the sheet from VBA is time-intensive itself, so of course it makes sense that if I had two calls to the sheet, it would take twice as long to run the function as if I have just one. But this issue where the processing speed decreases as time goes on leads me to think there's some kind of garbage collection or memory allocation problem that I hope can be solved.