Does it have to be a scan? Maybe it can be done with an SQL command?
Are you sure that SCAN itself is responsible for slowdown not what you're doing inside the scan?
Are you setting order in the table before scan?

>I have a table XXX with 25 fields (ctype, nNum, ... filed25) and aprox 100.000 records and following code:
>
>scan for ctype=cvar1 and nnum=nvar2
>...
>endscan
>
>
>Recently this scan became very slow. I've indexed the table in cType and nNum to optimyze speed but it does no work...