Larger layer time causes visible line on external perimeter

I'm printing what is essentially a rectangular box, with walls that are a lot thinner than the box is wide (~7 mm wall thickness, box is about 70 mm at its widest). I would like the inside of the box to be nice and smooth so I perform ironing on the inside bottom of the box. This causes the ironing layer to take significantly longer than the surrounding layers (20 minutes vs. a few seconds), and the result seems to be a very visible line on the outer perimeter of the box, where it consistently overextrudes in some places. Turning off the ironing seems to stop the issue. I know this is a relatively common issue, but I've tried many things and nothing seems to work to get rid of that line. Here are some relevant print settings: Printer: V-Minion, LGX Lite + Rapido 2 UHF Nozzle: 0.4 Material: PETG @ 225 oC, also tried 235 oC with similar results Ironing flow rate: 15% Spacing between ironing passes: 0.1 mm External perimeters first: Off Scarf Seams: Off Perimeter speed: 200 mm/s External perimeter speed: 150 mm/s Infill speed: 250 mm/s Ironing speed: 15 mm/s Pressure advance (recently tuned): 0.09 I'd be happy to provide any further info that would help the situation! Fixing this has been driving me up the wall... I've also attached a photo of the defect, as well as a print time analysis of the part from Prusa Slicer 2.9.2. I'd be really thankful for for any help you can provide! ❤️ Cheers!