j ai un iMac i7 fin 2015 c est devenu trop lent

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"I have an iMac core i7 at the end of 2015 but it has become too slow, is there a solution?

For my 2015 21.5" iMac retina, I switched to an external SSD to boot from and run the system. Made it a joy to use.

Use an external SSD as your startup disk … - Apple Community

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