PL/SQL :: Lag Function Cost Almost All Of CPU Resource

select *from v$version where rownum=1;

BANNER
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Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production

prepare data:
CREATE TABLE a AS SELECT LEVEL ID ,decode(mod(LEVEL,10),1,'CODE'||LEVEL) CODE from dual CONNECT BY LEVEL<20001;

sql1:
CREATE TABLE TEST AS
SELECT ID,
NVL(CODE,LAG(CODE ignore nulls) OVER(ORDER BY ID )) CODE
FROM a;

the sql is slow,most of the waits on session is resmgr:cpu quantum and almost all of the cpu resource is occupied

sql2:
CREATE TABLE TEST AS
SELECT ID,
NVL(CODE,last_value(CODE ignore nulls) OVER(ORDER BY ID ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING)) CODE
FROM a;

this sql is fast

How lag function is implemented if the offset is not deterministic?