The problem is that when you tell the extruder to stop extruding plastic, a little bit oozes out. You will get the greatest improvement by increasing the difference in speed for movements versus printing. So the suggestion of 40mm/sec is great because the default movement speed is 150mm/sec.
Another fix is to use retraction (which will drastically slow down your print. The settings in Cura set the distance travelled before retraction (retraction of extruder) is used and how much to retract the filament. You could reduce the distance setting to be smaller than your hole sizes.
The final fix is to lower your nozzle temperature a little so there is less oozing. Every color of PLA prints best at different temperatures so it's hard to recommend a temperature. If you lower the temperature too much and print too fast, you can damage your ultimaker with too much pressure in the print head. Ideally you should print a series of 10X10X10mm hollow cubes at different temperatures with same layer thickness as you plan to print most often, lowering the temperature by 5C at a time until you get an ugly look (tiny holes) due to underextrusion and then pick the lowest temperature where the cubes look perfect.