Hard to say,
This depends mostly on the number of peripherals connected to the Avalon bus. For every peripheral, an entrie is added to the input mux from the connected master. The larger the mux grow, the slower it gets.
You can have a look at the compilation report in the timing analyzer entry. If it says for example 73.5MHz, try with 80Mc as clock source. The compiling process will do more effort then to try to meet the timing requirements.
