depending on the BSC design (different CPUs handling different functions) BSC CPU overload can negatively affect a wide range of KPIs. The BSC will in most cases be designed to discard some calls when the CPU usage hits a predefined threshold.
Basically Call Setup will be negatively affected
KPI measurement some of them are set to collect automatically while others needed to be activated. activating/adding a lot of KPI measurements may effect the CPU load of the BSC. If you are experiencing CPU overload more than 70% then you have to think a way to reduce the load such as deleting unnecesary KPI measurements, disabling intra-cell handover,may be you have a too large Location Area Idenfication which may cause too many paging messages and signalling. The reasons of CPU are so many and a lot of KPI measurment is one of them.