Some of them have very bad optimization (i.e. a lot of unnecessary details which is completely unprofessional - you turn around at 360 degrees and there are some parts which make your fps drop to 70).Sadly mappers are not professionals. They are people that are willing to spend a lot of time to create maps for free for this community, and I dare say maps are quite important in this game. Creating an optimized map is extremely difficult, as you have no idea how it will run with loads of players on. Especially if you do not have a bad pc yourself it is really hard to estimate how far you can go before people with crud pc's start to suffer. Furthermore I think there is still quite little insight in what actually hampers performance, the do's and don't's for a smooth running map. I can speak out of my own experience that you are basically clueless to when performance gets bad
Good example is this map:too many unnecessary details which lead to enormous lags. I have a good machine (8gb RAM, 4gb radeon 7990) and i believe that the majority of this community (especially the eastern part) has way worse PCs.(click to show/hide)