My thoughts:
-There's no need to set PT samples to 40. You shouldn't probably use more than 25 and usually the default value is enough (right click to reset to default). With such high amounts of PT samples it will take much longer to produce nice clean anti-aliased edges.
So it goes like this:
[more PT samples] = better GI, less GI noise, noisy textures and geometry edges
[less PT samples] = worse GI, more GI noise, smooth textures and anti-aliased geometry edges
-Lights samples multiplier set to 24 is just pointless. I think in most cases there is no need to set it higher than 4. Actually setting it to a very low value like 0,1 sometimes helps to clean secondary GI faster.
So:
[more LSM] = clean edges on shadows from direct light (like sun or Corona lights), more noise on GI
[less LSM] = noisy direct light (from sun or corona lights), faster and less noisy GI
Setting LSM to 0 makes it almost equally noisy in direct light and GI parts.
-You can decrease max sample intensity, but to no lower than about 20. It will always reduce noise at the cost of tonal range.
-In your materials (especially on walls and other big, flat surfaces) never set albedo (diffuse amount) to more than 0,75. 0,5 should be enough. Making materials too bright always adds noise.
-In HD cache settings increase PT samples to 1024 or more. It will take longer to compute but should remove some noise.
Something that is still missing in Corona is some kind of physical scale for lights and camera exposure. You have to guess how bright your lights should be but never make them too bright. It's better to make lights and materials darker and adjust brightness in "post processing" tab. You can also make your scene with low contrast (brighter night sky, darker indoor lights) and increase contrast in post processing (for example in Photoshop).
I'm still not sure if this will work but I hope so. I'd suggest you reset all the render settings to default and check if it won't be faster like this.
