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I'm running the i7 4 ghz iMac with 24 gigs of ram and I'm assembling 4k and 6k timelapses based on thousands of individual 6000 x 4000 32-bit frames and I'm not getting any lag.

Example of the 1080 version:

Although it might be helpful for us to understand what you are describing as lag - are you referring to applications that are running a process you must wait for (progress bar) or are you actually saying you experienced the entire computer locking up periodically? I've never experienced that on this Mac.
 
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