The problem was not with wheel slipping but with the loco taking more time to accelerate on gradients (which in layman's term called 'struggling'). LPs had to go for the 8th notch (highest notch) to make it accelerate at lower speeds on a 'steeper-than-moderate' inclinations. There are plenty of gradients in and around SBC (in SWR especially).
BTW, WDP4s too have got the creep control capability.