If you're doing strictly sequential playback of music tracks, you may want to try Foobar. Very lightweight application. May or may not solve your problem though. Once WMP initializes, if the tracks in a playlist load slowly that may have more to do with your PC (hard drive speed, available RAM, gak running in the background,
etc.) than it does your playback software.
For actual sound effects cueing, routing, sequencing, and triggering, you could take a look at
MultiPlay as linked above. I've only used it for one show and seem to recall it was a little user-hostile, but that was 5 years ago and I've blown out of the back of my head whatever memory of what I encountered for that show.
What a program like
Qlab (for
Mac, and not for free) does is it'll load upcoming content into RAM so the moment you hit GO, it's accessing memory instead of spinning up a hard disk. I'm not sure that
MultiPlay does this, but if you have large files you're trying to
play back, that is kind of feature you'll want. For this reason, if your PC hardware is the bottleneck, you may have better luck playing
MP3's than WAV files because of the file size. If it's a slow hard drive or if it's the same hard drive your
operating system is installed on, you may also have better luck playing back off of a USB flash drive (preferably USB 3.0).