I think BB is a great game, aside from the fact that it lags horribly on 80% of the servers. I can think of a few ways to change that without going to Source:

-The blood and hit animation looks good, but has WAY too many particles. Reducing the number of particles and making some bigger "chunky" looking ones may help, as well as looking more realistic. Granted, some people like to see blood and gore, but the massive spray of like 30,000 red particles from a zombie's head is overkill.

-All the player/zombie models are so highly detailed that the sheer amount of moving polygons burdens the server. Again, reducing the polygon count and maybe simplifying the zombie damage system would improve performance.

-The levels are so highly detailed, and so big, that it becomes a major performance issue. Simplifying the level details to about the level that HL1 was at would probably be a great improvement with no significant loss of eye candy.

-Since zombies are dead, and dead blood will eventually congeal, wouldn't the blood splatter pattern look more like that of HL1? This would be simpler and would still look good.

Please do not start a flame-war.

1. The blood particles are controlled blood_low, blood_med, or blood_high.cfg file in the brainbread\partsys directory. I don't think this has any effect on the server since it appears to be controlled client-side, so you can control the blood particles to your liking. I turned up the amount of particles.
2. Each zombie model has 1128 drawn polys. As a comparison, the original LD Half-Life enemies and npc's are roughly 750-850 polys. The high-definition pack for Half-Life has models around 1500-1800 polys. BB is within a good range.

The damage system shouldn't be affecting performance much. In fact, in terms of model detail, it's helping. The poly count on zombies goes down every time they lose a body part. A zombie reduced to a walking pair of legs is only 272 polys.
3. As for the blood decals, how would a different set of decals improve performance?