Muse reads objects into memory as they're used and currently does so (mostly) one object at a time. Since each file access is many many times slower when the .muse file is on a server versus your hard disk, I believe that explains the performance you were seeing.
We're working on revising how we load objects into memory to improve performance in general for a future Muse release. That change should make working directly from a server more viable, but working from a server will always be slower than working with the file stored on a local hard drive.
As for image size, you shouldn't need to reduce the files outside of Muse, though you certainly can. There's a relatively little known feature in Muse in the Assets panel called "Optimize asset size". If you right click on an image in the Assets panel this option will show up if Muse is currently storing more image data for that image than is necessary in the .muse file. Specifically, if the image is currently being used in your Muse layout at a smaller size than what was imported, then this option will be available. If you select this option, Muse will discard the excess image data. (It will store an image that's just the exact size that's being used.)
If you select the first item in the Assets panel and shift click to select the last (to select all), right click and choose the "Optimize asset size" option (which may take a while to run), then Save As to a new file (since Save As compacts file size and so the original file is available to compare to) you'll likely find the new file to be much smaller than the original.
The gotcha with the "Optimize asset size" option is once it's been done, all the images stored in the .muse file have been resized to their current size in your layout. If you then resize an image larger within Muse, it will become pixelated because it's now larger than the stored image. When this occurs Muse will warn of "upsampled" images and a little icon with a plus will appear next to any upsampled images in the Assets panel. If you right click on an image in the Assets panel that's upsampled the context menu will include an option to "Import larger size". Note that for this option to be enabled the asset has to both be upsampled and the link to the original file needs to be intact (so Muse can re-import the image data).
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