Inkscape can export scaled bitmaps. Besides resizing, the resulting graphics can take into account added enhancements such as clipping, masking, gradients, and filter effects. If density and dimension are final, the original graphics and part of their elaboration can be replaced with the corresponding exported bitmaps. That makes it possible to continue working on a document using less memory and less processing power.
Each image has to be arranged manually at least once. Create an invisible rectangle around a raster object and export the visible area to a file. At 90 dpi, each rectangle's bitmap size matches its export area size in pixels. The exported file can then be reimported. Repetitions can be made easier. For example, you can put all invisible rectangles in a layer named "exports" and hit Ctrl-A to select them, then Check "Batch export N selected objects". The corresponding visible objects can stay in a different layer. Yet another layer can contain links to the exported bitmaps. Batch export silently overwrites files, so they are automatically updated if you keep reimported image externally linked (rather than embedded). That way you can work on a single document by switching layers visibility and lock status.
Instead, if the intent is just to obtain a reasonably sized, shareable document, it is much easier to save a copy as PDF and optimize it using an external tool, e.g. Ghostscript.