That's right, Bill. Imaging time is increased because less light is getting through the filter. That's the downside of an LP Filter.
The upside is that the signal you DO capture is desirable light with all the nasty wavelengths filtered out. This dramatically reduces luminosity and color gradients inherent to urban astrophotography.

Hi, Kevin,almost every single image on my site was taken with a light pollution filter. The light pollution is so bad at my observatory location that I reach 1/3 histogram and begin fogging at about 30 seconds at ~ 500mm f/5.0.
The Orion Skylight Filter I use blocks common light pollution sources (mercury vapor, etc) and allows me to increase my exposure times up to around 3 minutes.

Maybe you can borrow one from a fellow club member to try out. If you're in an urban setting, you will be impressed with them.

That's exactly right, Bill. But the upside is that only the light you want to pass through, is the only light that is passing through. All the extraeneous light you DON'T want is getting blocked by the filter.
This also makes focusing much more difficult, because the image is darker.