ok lets start,
1. search for the skillID (f.e. Acumen ID = 1085)
2. open gameserver/data/stats/skills (f.e. for Acumen open 1000-1099.xml with notepad)
3. scroll down till u find the skill :
</skill>
<skill id="1085" levels="3" name="Acumen">
<!-- Done by dc, completed? by NewAge

-->
<table name="#mpConsume_Init"> 4 6 9 </table>
<table name="#mpConsume"> 16 24 35 </table>
<table name="#Tab-mAtkSpd"> 1.19 1.27 1.35 </table>
<set name="mpInitialConsume" val="#mpConsume_Init"/>
<set name="mpConsume" val="#mpConsume"/>
<set name="power" val="0.0"/>
<set name="target" val="TARGET_ONE"/>
<set name="hitTime" val="4000"/>
<set name="reuseDelay" val="
6000"/>
<set name="skillTime" val="
4000"/>
<set name="skillType" val="BUFF"/>
<set name="isMagic" val="true"/>
<set name="operateType" val="OP_ACTIVE"/>
<set name="buffDuration" val="1200000"/>
<set name="castRange" val="400"/>
<for>
<effect count="1" name="Buff" time="3600" val="0" stackOrder="#Tab-mAtkSpd" stackType="mAtkSpeedUp">
<mul order="0x30" stat="mAtkSpd" val="#Tab-mAtkSpd"/>
</effect>
modify the values i marked in red to get a faster casting for the spell
lower values = faster
values are displayed in ms