I see 2-3 remaining things that could be the source of the problem:
1/ The tilesource “dimensions” (number of tiles)
I have a feeling about this one…
=> When I switch from 32x32 tiles to a 16x16 tiles, the tilesource image was smaller BUT the tilesource had the same number of “tiles”… this may explain with the collection didn’t load faster.
=> A few weeks ago, I removed elements from the tilesource image, made it smaller, reduced the tilesource dimensions in the process… and the collection loading time was reduced by like 3-4s… At this time I thought it came from the tilesource image size, but in the end it may come from the tilesource itself ( the nb of “squares”)
So I’ll try with a smaller tilesource and… we’ll see. This is my last hope. 
2/ Monarch 
(I hope not)
3/ Defold (I hope even less 
)