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Jo69 - I'm not sure what you did, so I'll spell out what I was suggesting i detail. Firstly create a column like this:
Then set up your column icon rules, like this:
Your custom column would have a name such "have_read/Have Read", and you probably don't need an icon for 'N' or Undefined.
Why am I suggesting you try this ? Its a long shot, but . . . attaching an icon to a Yes/No column is a bit oddball - it already has icons, the green tick and the red cross. So, I'm wondering if overriding those icons is what's making it slow on your system.
BR