Same problem here. I compared my phone with another one, Xperia SP, side by side and Mini wasn't able to lock at all (indoors and poor weather conditions) and Xperia SP locked in 10sec. and signal was very good, considering all conditions. Then i opened my Mini and pry all connectors and tried again. First time after boot it took to Mini about 1,5 minutes but after that Mini was able to lock position very fast. Signal is still not strong like on Xperia SP, but it's good enough that we can say it's decent and very usable and since i use gps mostly in my car i can say that now is pretty good. So, Likeadracula, thanks for the tip!
Edit: GPS signal is strong enough but on every 5 minutes or so is lost! It works very nice and then just vanish like someone turn it off. It's not like signal weakens and weakens and then it's gone, no, it's good signal all the time and then just cut! No more signal. Is it possible that is software problem or maybe gps.conf file?
Edit2: After testing i can say that my phone while it was on 5.0.2, v20, GPS signal was strong enough, but strange thing happening, every 5 minutes (no more or less) some kind of reset happens and signal is lost, then after 30-45sec. signal is back and working nice. And every 5 minutes same thing happens. Then i downgraded my phone to 4.4.2, v10, and now i don't lose signal like before, no resets, but reception is (no doubt) weaker. But good thing for me that no reset, so i will now try to make better signal like SkyLK did it, with CU foil and i hope that will make reception good enough.
Just, it is strange that there is lot of difference in signal just because another firmware. I don't get it.