Slow Upload from CF Express Card or SD Card

When I upload from an external Card via a Card Reader to my iMac it takes a long time to transfer files up to half an hour for around 300 files. When I use the same Card and Car reader to Upload to Macbook Pro the file upload takes a couple of minutes or less with the CF express card. I have tried using USB and Thunderbolt connections on the iMac and it makes no difference. I guessing the issue lies with the iMac. Has anyone got any thoughts or suggestions what might be causing this ? Thank you