I'm quite new to espresso, but have tried to educate myself. This is something I need an opinion from more experienced people.^^

I use: [Quick Mill 3035]http://www.quick-mill.nl/quick-mill-303 ... hine-4434/ with a two shot portafilter. I use good quality beans.

After grinding and tampering my extraction is as follows:
30 ml in 17 seconds with pressure 10 bar. Some shots are even quicker 13 sec with a pressure of 8 bar.

As I understand, the optimal pressure is between 8-10 bar and extraction time 20-30 sec. So, pressure seems ok, time is too quick

What can I change, do or believe?

Grind finer.

If you tried to educate yourself, you probably know the importance of a consistent grinder to get good results. What kind of grinder are you using?
As Ellejaycafe said, grind finer to get the flow rate right. Use a scale to weigh your coffee grinds and get 2 - 2.2 times that amount of espresso in 20-30 seconds, then you're in the ballpark.
You will notice that the pressure will increase as you grind finer. Don't obsess over that. What tastes good is important.

It could be a number of variables. Make sure you're dosing the right amount for your basket. Try to find out the recommended dose for your basket and stick to that. Fining up your grind will (most likely) help. Your tamper could be too small for the basket. Your temperature could be too hot as it will sometimes speed up shots as well.

Let us know if you have further questions.

I use the grinder of the QuickMill machine. I'm told it's of good quality. (QuickMill is also a grinder manufacturer). Maybe not the quality of a $1000 grinder, but I hope good enough.

I use a two-shot portafilter and my coffee grinds weight 14-15 grams.

The tamper is of the correct size.

I shall try to grind finer and ... just ignore the pressure shown on the display?

Some double baskets can range from 14g to 18g. Don't be afraid of going outside the 15g range.

Ignore the pressure gauge. A lot of gauges (even on commercial machines) don't read very accurately. It's something to keep in mind but not obsess over.

I set the grinder finer. Adjusted the amount a bit, until I got 14 grams again.

The extraction time is now in the 24-28 sec range with a pressure of 11-12 on the display. (@41fivestar, Good to know that these readings are not exact).

The espresso weighted 28 grams.

When I judge the stream, it's more oily like in the beginning. The taste is definitely stronger.

I can't judge if it is a good taste, because I'm not a coffee drinker. My wife is (it's her machine), but she drinks cappuccinos. Nevertheless, I like the process, therefore want the best quality I can achieve. Unfortunately, I have to go by the numbers.

Thank you all for the advice.

Good to keep in mind, like already stated, that the gauge generally isn't correct. On my commercial machine; a showing of 10 bar on the gauge is a little lower than 9 at the puck. It's best to go off taste and adjust from there when it comes to pressure.

Don't forget how much fresh roasted coffee will affect taste and flow...if it's stale it'll flow fast...and taste horrible, well not as good as it should