[ic] How to reduce load caused by search

[DB]

My site runs IC 4.8.9/foundation/mysql

I'm using the code below to generate an "index" page that lists the 
products in my catalog (over 250,000 items). I'm having 2 problems:

First, the search is causing way too much CPU load. I think the main 
problem is my use of the "fm=..." in the search.

Second, since this search causes such a load I'm trying to use 
timed-build to reduce the need to run the search. However the server 
only seems to "remember" the search results for a finite time. I want 
the results to be cached forever, or until I manually remove them.

Can anyone provide tips to decrease the load caused by the search, or at 
least tell me why the timed-build results are not "permanent"? Here is 
the code:

==========================

[timed-build auto=1 force=1 minutes=0]
[search_region more=1 search="
                  ra=yes
                  st=db
                  fi=products
                  tf=sku
                  fm=501
                  ml=500
                  "]
[search_list]

<A HREF="http://__SERVER_NAME__/[item-code].html">
                 [item-field sku] </A>: [item-field description] 
[item-field comment]
<hr>
[/search_list]

<br>

[/search_region]
<small>
Built [time]%c[/time]
</small>
[/timed-build]

=======================

Thanks for any hints.

DB