Data Thinner code seems very inefficient- used on large data sets

Good afternoon!

I have a file with two worksheets of data, one with 1000 lines and the other with 15,000 lines but both with 3 columns across. This is just a sample of the files I deal with, most run around 100,000 to 150,000 lines of data (three columns worth). I can manage these plots because my system has some power to it, however some other office systems do not have the same power and would like to run this plotting workbook I have created.

I really want to thin this data from 150,000 to 15,000 (or something of that nature) and I figured I would write some code up to try and remove 9 of every 10 lines of data keeping that last one and moving to the next bundle of 9. This is what I came up with...

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I thought I was being pretty clever but realized that even at 15,000 it is taking WAY too long and if people start using it for the 150,000 row file it would take over 10 mins (thats just on my computer).

What am I doing wrong? Is there a way to thin the data better than what I am doing now?

I have attached a stripped pile of data on sheet1 and sheet2 of the Data Thinner workbook with my macro inside if you are interested in looking at it.

Thanks in advance!