they are useful only if the feature IMMEDIATE ASSIGNMENT REJECT is activated.
those are important timers, they force the MS to retry a CH REQUEST on W_xxx later, instead of almost immediately.
Those timers are used only when the cell suffer SDCCH blocking. So they don’t improve the “delay” of Call setup, but they allow it in the first place !
If you reduce these timers, you reduce the chance of successful CSSR, and you increase the sDCCH Congestion.