You are probably hitting a memory limit. When your application cannot get enough memory it will switch to disk cache. You should see an increase in disk IO at that point. Are you using the Q8 version or the Q16 version of Magick.NET? The Q8 version might be enough for you since you are already calling the method Quantize. Switching from Q16 to Q8 will half your memory usage. You should probably also limit the amount of threads to a value that will allow you to keep everything in memory.
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