Hi all,
For those of you affected by lag in the replaygain script, I have made a new release trying to address that.
In adition to installing the new script release, have a look at the

Lag reducing howto
Lag seems caused by amarok itself by taking too much time to return from dcop calls.
This has been related to high cpu load caused by sqllite and cover image rendering.
So, if you are affected, what can you do?

• The cpu load for using sqllite has been reduced in 1.4.0 and later, and if you are using 1.3.9 you can apply the patch included in the amarok replaygain script package. Install either of those
• Install kdebindings3-python to take advantage of new script's code.
• If you are using the script's OSD control (the "Disable OSD during...." option) disable the OSD or uncheck the script option, it couldn't be optimized because of bugs in pykde
• To see how cover rendering affects you, you can move your covers elsewere (try with an author or two) so they are not found. Large cover files may have a big impact, if this is the case, use gimp to reduce them.
• Run amarok minimized in the tray, it will not update visual elements saving much time.
• Resource intensive software affects too (some screensavers, p2p, ...)

During this beta release debug output is enabled. Have a look at your lag (in seconds) by typing in a konsole/xterm:
tail -f ~/.kde/share/apps/amarok/scripts-data/replayggain.log|grep time
As a reference, my lag is well under 0.1 s (usually 0.01-0.05)