slow mysql "sending data" stage between mysql and php

Hello!
I have problem:
I have IPB forum installed. After search in IPB (search takes about 3-4
seconds for post table about 300 000 records) mysql shows "Sending
Data"
status and takes about 3-5 minutes.

I tested with PHP 5.2, PHP 4.4, PHP 4.2.3 - with same results.
I switched from MySQL 4.2.18 to 4.2.21 and 4.2.22 with same result.

I tested with apache module, fastCGI with NGINX web server - same
results.

I tested mysql connections with socket and network connection.

May be I missed some variables from php.ini?
May be I missed some variables from my.cnf?
Main question - how I can speedup code for accept "Sending data" stage
between PHP and MySQL more fast?