I've been playing on this server a while now, and I'm one of the admins.
The 10% clear is about right. If your group is good, is does not feel that way at all, as you can easily clear several maps in a row without breaking a sweat. The problem is that this server is open to everyone (level 5 and above). This isn't always a bad thing, as they are some amazing players out there, and you always need new players to replace the ones that leave. However, not every player is equal in terms of skill, and most importantly in Friendly Fire games, teamwork.
It only takes one bad apple to ruin a good attempt. The few players who put themselves before the team, and believe they are hot shots because they are getting the most kills. The players that refuse to stay with the group, because they get more action out in front. The players who think that getting the last few kills in a wave when everyone is heading towards the trader makes them the best. The players who stand slightly in the way, so that the experienced players behind will not actually hit them, but they end up limiting shots. These are the types of players who will lead to group wipes, and they won't even know they are doing so. Since they are never "actively" the cause of a wipe, it is hard for them to see they are the reason their team dies.
Ideally, the best group would find a good firing position, and then never move. They would leave the last 5-10 specimen up, head to the trader, and kill them when right in front of the trader door. They would limit specs that are less than ideal, based on the map. This is the hardest one, as people enjoy playing certain specs, even if it is not "the best." I don't fault people for this one, but sometimes, if you want to win, you shouldn't be the 2nd or 3rd or even the FIRST medic on a team that takes 0 damage. You shouldn't be a support on a map that 90% of the specimen don't make it close enough to shoot. You shouldn't be a demo if the area you are shooting blocks line of sight for everyone. I'm not saying these specs are always bad, but in certain situations, they are. Accept this. If you realize you are not pulling your weight, change spec.