I did a bit of research and could not find an awnser, so here it goes. Does having more slots on a dedi server reduce the preformance of the server?

Do you mean just having more slots available or having more guests? If you just increase the number of available slots but keep the same number of users I can't see that decreasing server performance. If, however, you increase the number of slots and new guests join to fill those slots there will be an increase in resource utilisation as a result of that. The largest increase in demand is likely to be on the internet connection (a quick guide on bandwidth requirements is available here in the LFS Manual). There will also be an increase in the CPU and memory demands, but that shouldn't be significant.