On the graphical side?
Tilesets are a lot more efficient, even with processing and memory usage. Take a 13x16 tileset (assuming each tile is 16x16)...that tileset is only generally around 24 kb while each tile on it's own would only be 4 KB...but 4 KB multiplied by 208 tiles would probably take up a fair bit more space than 24 kb. 832 Kb but still, it can add up if you have several tilesets that are more than 512x512 in size, I mean comparatively if you were to use several amounts of 512 singletile sprites.
I'm not sure about the sound files, though.
Are you using the recommended audio filetypes with Construct 2, i.e aac/ogg? I don't think it would effect compatibility but you could try including only ogg files. I'm doing that with a project of mine (well, initially inadvertently) but my game seems to run fine on both NWJS and browser.
Besides that, I'm not sure what else to suggest. I mostly only know visual/graphical optimization, as for why. I'll try looking into more tips for you, though.