Paul,
The uncompressed 16 bit TIFFS are from Canon 1DMk4, 91.5Mb. Photoshop CS5 uses 75% of available RAM (total 8Gb). PM "Generate High Quality Thumbnails" is checked. Thumbnail size is slightly bigger than default.
I just timed PM taking 5 seconds to respond to a Tag (T) command in a folder of around 250 TIFFS.
Were you tagging one photo, or all 250 of them?
I've just worked through a folder of just 16 TIFFS and PM functions were instant. It looks like PM chokes on big folders of TIFFS, even after it's been open in Contact Sheet for +20 minutes.
This is likely because your system's disk cache has nearly all of the data for each file still in memory.
My experience with Breezebrowser Pro is that it hardly blinks if I throw a folder of several hundred TIFF files at it. Rendering, commands as basic as Tag, Rename are done in Olympic qualifying time. I expect PM will match this when properly configured.
How long does BBPro take to set IPTC data on several hundred of these large files? I don't mean just tagging, but setting an IPTC caption.
Rename should be fast on PM as long as you're either not using variables, or if you're using variables and PM has had time to get metadata (EXIF, IPTC, XMP) from the files prior to the rename command.
Tagging should be fast on PM as long as you haven't told PM to put 'soft edits' in IPTC/XMP for all files. In that case, PM just has to update the end of the file. If on the other hand you've told PM you do want soft edits written to IPTC/XMP for all files, then PM has to modify/embed metadata into the middle of the file. This takes much longer as the file needs to be expanded in the middle of the file. Many megabytes need to be moved so that new space is created. This same process is done when any of the soft edits are made with this preference set (tagging, rating, color class, and cropping). Once this expansion is done, further updates shouldn't take as long.
Here's a screenshot of the PM cache prefs.
I'd definitely set your Disk Cache to something larger like 1024 or 2048 MB. And as far as Photoshop goes, does it really need 6GB of your RAM? That doesn't leave much room for your OS or other apps. I think 50% should be quite adequate for Photoshop's needs on your system (8 GB of RAM).
-Kirk