Slow it down, no. Increase fuel consumption yes. Also most cars sold by manufacturers today have steel rims (if it isn't alloy it's steel, they are easy to spot). Alloy rims are significantly lighter so you can increase the size without compromising weight.
Going from 14" to 17" your car will definitely be slower. Use a gear calculator and you'll see the huge difference in speed between 14" and 17"
If the Polo has steel rimz, the 17 alloys will probably weight the same.
And btw, theres probably about 5 cars that come with steel wheels these days.
As for the speed, the first month there might be some drag, gear ratios will feel different, 2 - 3 months, 1000km later, it will all be back to normal.
Im speaking from experience here. Although Ive never owned a 1.4.