Calculation Time Not Scale With Worksheet Size

I have a rather large spreadsheet that takes a long time to calculate. One sheet has the data (about 2800 rows by 650 columns), with samples in rows and data for each sample in the columns. A second sheet has functions to group the data according to predefined patterns. That second spreadsheet is about 800 columns wide and as many rows as I need it to be, as the samples don't need to be analyzed all at once.

Ideally I would like to do all samples at once by making the second spreadsheet 2800 rows high, but the calculation time is just way too long. And there's the problem. The calculation time seems to increase exponentially with the number of rows I calculate at a time. I ran a few tests making it different sizes, and here are the results:

rows
time(seconds)

100
3.5
150
12
200
23

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I tried both HLOOKUP and INDEX/MATCH functions and it didn't make a lot of difference. These numbers are for INDEX/MATCH.

Graphing these numbers show that it follows an exponential curve pretty closely. What is interesting is that the progress indicator at the bottom of the screen finishes in a few seconds, about 5 seconds for 350 rows, and I can see the new data after those few seconds. But then it sits and appears to do nothing for the rest of the time. So, for 350 rows, it looks to be done with the calculations within 5 seconds, but then it freezes for another 2 minutes with CPU usage maxed out before I can do anything with the program. That actual calculation time seems to scale linearly with the number of rows I calculate, but the time after it finishes calculating before it finishes whatever else it's doing scales exponentially.

The spreadsheet does have a macro and a couple of other sheets and a macro that do some later analysis and I wondered if that had something to do with it. To check that, I created a new spreadsheet and set it up the same as the original one, with the data in one spreadsheet and the INDEX/MATCH formulas in the other. I copied the data over, but setup the formulas new without copying/pasting anything. It didn't make any real difference.