Number of players on screen
Nothing is wrong with you computer. This is called culling, and it is bad for everyone. Oftentimes during large events, important enemies will be culled (made invisible), making it impossible to tell what is hitting your or what you are attacking. The system is in place in order to try to reduce stress on computers and on the server on which you are playing, but it is poorly optimized and we have had yet to hear of a solution being in the works.
Culling is more certainly not bad for everyone. The game is optimized to load only a certain number of character entities (humans, npcs, mobs) and is prioritized based on importance (npcs, party members take precedent) and then distance. WvW just was updated to have a separate culling limit for enemies as it does for allies.
While this process is currently not configurable, so it over culls for high end machines, it prevents low to mid range machines from dropping to a crawl in high density areas. Without culling unless you had a super high end machine your framerate would slow to a crawl in large scale engagements.
ANet is constantly working on improving the system and I suspect it will eventually be a lot better than it is now and also somewhat configurable.
Okey thanks, im glad to know that im not the only one with this problem. I know nothing about stress on servers but my rig sure could put up with a lot more..