does this module slow down the site

Do you understand the difference between client-side and server-side performance?

It's true that using the lightbox javascript will slow down each page load/responsiveness on each browser page render (client-side) somewhat depending on your page weight and local CPU. This can be mitigated a little by choosing better jquery element patterns in the UI, but depends mostly on your design/theme.

If you have found that lightbox is having an effect on server-side performance, please try benchmarking the server with and without lightbox enabled. I don't think this is really the problem, but if you can demonstrate it it would be interesting.

I suspect that what you are (very vaguely) describing is mostly about the difference between a simple HTML page and a complex dynamic page doing .lots of javascript magic on a slow computer. It is an added, advanced feature, and will make client-side performance hurt a bit on old computers.

Can you explain what you mean when you say the 'performance' is down?

It is slow, but I think the main problem is the server speed. I don't think there's much related to lightbox itself.
I looked at it with http://developer.yahoo.com/yslow/ and
http://www.websiteoptimization.com/services/analyze/index.html

Strangely, it seems the css requests are single-threaded. Each css file is loading one after another instead of all together. This means it takes over 10 seconds before each page loads at all.

You should turn on css aggregation to reduce the number of requests. And there's a couple of 404 errors that are wasting a bit of time. But according to my connection it's mostly about download performance from apache. So it's probably up to your choice of host. Sorry, there's no quick fix for that.

I've just checked your site and it seems a lot faster now, though I am using a broadband connection which you may not be. I'm gonna go ahead and mark this issue as fixed. Dman has offered some good advice above, and I agree while lightbox2 will slow down each page load/responsiveness, it is unlikely to be the main source of the problem. You should look at your hosting and apache configuration.

If you can demonstrate that lightbox2 is causing a significant decrease in performance, please feel free to re-open the issue.

Cheers,
Stella

Use the "Page Specific Lightbox2 Settings" configuration option on admin/settings/lightbox2

Cheers,
Stella

Enter in the list of pages that use the image module and select the "Load only on the listed pages" option. For example, if you know that image galleries are always : http://www.example.com/image/tid/XXX where XXX is an id, then you can enter image/tid/* as one of the paths and then image gallery pages will have the lightbox2 javascript files loaded on them. Do something similar for the other pages, or list them one by one if necessary.

Cheers,
Stella