WMP was so painfully slow that I switched to Media Monkey. Basically, I needed a decent tag editor. Media Monkey is not only faster at updating id3 tags, it also is easier to use and more powerful.

I suspect that if you have a setting telling WMP to update the media library on launch, this could be causing the delay.

Tools > Options > Library shows that WMP automatically checks My Documents > My Music and My Documents > My Videos to update the library. If you have a lot of files in there, that could cause it. Either move the files or point WMP away from that if you don't use WMP for keeping a library.

Other possibility:
In Tools > Options > Player
You can tell it to check for updates Daily, Weekly or Monthly. Perhaps that's why some days are quicker than others.