Issues editing older illustrator files in CC 2017.

I have a brand new, top of the line iMac and am running Illustrator CC 2017. I'm having issues editing illustrator files created in CS4-6. The artwork is not overly complex. But when I start editing the files they seem to slow Illustrator down, to the point where it crashes unexpectedly. When I am doing my own work, which tends to me far more complicated, Illustrator seems to run just fine. I assume this is because it was created 100% in CC 2017. Has anyone encountered this problem and is there any way to fix it? Any and all help is much appreciated.

Thanks,

Chris

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