In terms of I/O, the largest bottleneck is generally where the tag file is written. As Graeme suggests, adjust Workspace Properties to put the workspace tag file on a fast SSD or if you have enough memory, a RAM disk, and you'll see a big improvement.
Also, if you have enough memory, try adjusting Tools > Options > Background Tagging > Background Tagging Threads > Maximum number of active tagging jobs and Maximum amount of background tagging memory usage (MB). Give both of them an order of magnitude increase (10,000 and 256 MB), and you'll also see some gains.
The other thing I'd like to point out, and I see this a lot, is that if those 100,000 files are 10 copies of different versions of a set of 10,000 files, then, well, don't do that, it's just not good practice to toss everything into the blender. It just makes things harder for the tagging engine without providing you with any real benefit. Create a separate workspace for each version of the files.