read-only standby performance

I set up a standby database to use as for reporting/warm backup, and all works fine when I open it read-only for reporting, apart from the performance of the report queries. A reporting join query like

select a,b,c,d from x,y,z
where <4 conditions>
group by .. order by ..
takes less than a half a second on the primary and 25+ secs on the standby.

I would appreciate some tips on how best to investigate the source of the slowness.
This is Oracle 8.1.6 Enterprise on Solaris 7. During the query 'top' shows near-100% CPU usage and a little (1-5%) wait-for-IO.

I would normally use statspack to investigate what is slow, but it doesnt work on a read-only db..

I thought initially it was to do with slowness of disk-based sorting, so I created a second 'reports' user with a locally-managed temporary tablespace for that user, (but still doing queries on the first user's schema -would that use the first or the second user's temp space?) , but it showed no speed improvement. In addition:

SELECT name,value from v$sysstat
where name like 'sort%';
shows no disk sorts:
NAME VALUE
--------------- ----------
sorts (memory) 1830
sorts (disk) 0
sorts (rows) 454709
but that may be normal for a read-only db (?)

Any tips gratefully received. -please bear in mind that I am a Unix sysadmin with fairly rudimentary oracle knowledge.. Pls let me know if you need details of init.ora settings or whatever.

thanks
Robin

A couple things I thought of off the top of my head:
1. I assume your active database is being used by users. This keeps the cache fresh and some/all of your data may be coming from cache. On your standby, your cache is empty.

2. Maybe parameters, I/O subsystem, etc. are different on the standby?

an update on this, for anyone who sees it in the future:

stopping and starting the standby fixed the problem.

the only explanation I can think of is :
modifying a user's temp tablespace is a database-structure (DDL-type) change, and these don't get changed on the standby until a restart ? only DML-type changes get changed immediately on the standby (?)

mmm lots of learning to do..

anyway all happy for the time being !
thanks, Robin