Version 1.5.0 of the plugin has been posted to Codeplex: http://psdplugin.codeplex.com/
As you can see by the jump in version number, I consider this to be a big deal. Please report any bugs that you discover through the Issue Tracker on the Codeplex project page.
The biggest improvement is that RLE compression now works fully on save. File sizes are now competitive with Photoshop's. Say goodbye to those 100 MB uncompressed PSD files; your files are now small enough to email. Files saved out by Photoshop are still smaller, since it uses a better compression algorithm. However, the difference is very slight in most cases.
RLE compression does take some additional CPU time to compute, approximately doubling the time it takes to save a file. To compensate, loading and saving have been parallelized at a coarse granularity. So a dual-core machine will perform as well as with version 1.0.7 of the plugin, while a quad-core machine will be faster, period. And since loading had already been incurring the RLE decompression hit, it gets a performance boost even on dual-core machines.
The parallelization is not perfect. The granularity is coarse, and some sequential segments remain. The intention is to improve the parallelization of the plugin after Paint.NET has been retargeted to the .NET Framework 4.0, which includes the Parallel Extensions. Paint.NET has historically remained quite close to the cutting edge of .NET Framework versions, so this should not take too long. The plugin is not the bottleneck for loading PSD files -- Paint.NET takes much longer to render the preview images for the individual layers -- during which the application is mostly unresponsive. Thus, there is no urgency to improve the parallelization.
Further performance improvements in this version, independent of the parallelization:
[*:3jin6no7] Reduction in memory consumption when saving. Thanks to Rick Brewster for his suggestions on the code.
[*:3jin6no7] CMYK files now load twice as fast.
[*:3jin6no7] Lab files now load 3x as fast.