How to avoid negative slack?

If the clock is using global routing, it does not matter how many loads are on it.

Start with "Tools --> Advisors --> Timing Optimization Advisor --> Maximum Frequency (fmax)" in Quartus.

If none of the recommendations in the Advisor solve your problem, then refer to the Quartus handbook in Volume 2, Section III, Chapter 8 "Area and Timing Optimization".

If you still have timing violations after doing the basic things to improve fmax, then you might need to restructure your SOPC Builder system (for example, put some of the slaves on a bridge so that not everything is on one big Avalon bus).