Hi Mark,
Thank you for your reply. You are describing very well my problem. That's exactly it.
For this map, I don't have a dataset for a larger scale (i.e. zoomed out), so I did what you suggested. I've set a scale limits as such:
groupLayer.setMaxScale(1001);
groupLayer.setMinScale(10000000);
[side question: what does these numbers means exactly, do you know? I couldn't find the meaning of the double values in these scale methods, please?]
So, with the scale limits, the titles are not visible until I zoom-in now. Which is a bit more acceptable, performance wise. However, the downside is when zoom-ed out, the user have no clue that there are tiles there.. until you zoom-in.
I would be very interested to know more about the pre-processing method you are suggesting, please.
Many thanks again Mark!