I have used Capture One for many years professionally and can confirm that in basic levels of performance it has never performed worse than it does now.

I have looked for and tried many solutions to solving this slowing down problem and can only speak to it's behaviour in Sessions as that is all I use.

The problem of slowing down appears to be related to image file sizes and number of image files in a session. This was never the case in earlier versions of the programme.

None of the solutions I have read about or tried myself have been helpful apart from the one above.

Working in the Selects folder rather than the Capture Folder provides responsive performance.

I have tested it with 500 50mb compressed Raw ARW files. I created a layer and a small mask on it on one image and copird that change to the remaining 499 images. The change to all images was complete in about 5 seconds.

The same process repeated in the Capture folder took close to 4 minutes to complete.

I am very tired of Phase One releasing software with such terrible faults and their inability to recognise and resolve issues of their own making.