In both cases, not that I'm aware of I'm afraid. I've also never done either of these (well, I use Paint.NET, but I've never made a map only using Paint.NET), so I probably can't provide any advice on either.
There are AH.commers who make maps solely in Paint.NET, and of course there are AH.commers who make Wikipedia infoboxes, but I don't think they frequent this thread. You might have better luck posting in the map thread/alternate Wikipedia infoboxes thread respectively.
Ah, Inkscape tends to do that if you're making a complicated and detailed map, due to all the path nodes it's got to keep track of. The only way to actually reduce the lag would be to remove path nodes, which I assume you'd rather not do.
You could open your basemap image in a new file without all the bits you've already traced and trace the rivers separately in there, then copy the rivers and paste them in place (there's an option for that under the Edit menu, alternatively "Ctrl + Alt + V" works too) in the original file. It'll still take a while to copy and paste the rivers over, but you'd be able to draw them without any lag at least.
Edit: One other thing I will note is that - for some reason - if you zoom in then move your view Inkscape starts to lag more than if you just zoomed out and zoomed back in. I don't know why that happens, but if you zoom out and zoom in when you want to change your field of view instead of scrolling whilst zoomed in it can prevent a significant amount of lag in more laggy files.