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some questions about via optimization

Dear all,
I run via optimization on a full routed design at 65nm,using the following commands:

###step1,replace multi-cut vias with single-cut vias
setNanoRouteMode -drouteMinSlackForWireOptimization 0.005
setNanoRouteMode -droutePostRouteMinimizeViaCount true
routeDesign -viaOpt
saveDesign DBS/via_opt1.enc –tcon

###step2,single-cut via to multiple-cut via swapping
setNanoRouteMode -drouteMinSlackForWireOptimization 0.005
setNanoRouteMode -droutePostRouteSwapVia multiCut
routeDesign -viaOpt
saveDesign DBS/via_opt2.enc –tcon

I found the via optimization (step1+step2) degraded setup timing, then I checked the delay on a certain net on the same timing path in stage of pre_via_opt/via_opt1/via_opt2 respectively, the delay in via_opt1 is less then that of pre_via_opt, and the delay in via_opt2 is much larger than that of via_opt1.my question is as follows:

Q1,it seems that multi-cut vias to single-cut vias swapping (step1) will decrease the net load, then decrease net delay, and on the contrary, single-cut vias to multi-cut vias swapping (step2) will increase net load and delay,isn’t it?
Q2,why slack value of “0.005” specified in each step not prevent timing degrading?
Q3,as via opt degraded timing,I have to continue to optDesign after via opt,but it will decrease the percentage of multi-via, should I run via-opt again?
Q4,is there any command to get/calculate the percentage of multi-via, except finding it from log file

Thanks and regards!
William

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