Yep - I have been working on 3 of your worlds at the same time practically and playing the Sims of ML.. I made one world - the one without buildings an all circus and Showtime world. I turned the empty version of Legacy Island into a place for my fairies. i first thought all that cement might be annoying, but discovered it recolors well and what shows looks like side walks. I also had made a version of the big empty one for Fairies and Witches, but seems it allowed me too much room and all those downloaded wonderful Fairy type and witchy type houses and buildings, made it a bit laggy. After I put that one aside I figured go a bit smaller and went with Island Build. While doing so I think I discovered the culprits bugging Glenvale County and at least one of them is the very fantastic Fairy Village made by Elle. She has like every RH under the Village and the village on top also has a ton of shops including all the Ambition ones, and WA ones. I think it was just too much for one lot to handle or there is some conflicts going on with all those lots as I removed it in the Legacy and my bogging down instantly stopped. So I made something along the same line but didn't add any of the WA or Ambitions shops on the lot and used the Lunar lake portals in the cellar just for the needed RH's above it all I just had a huge gym with rock climbing wall, and the martial arts stuff in it and now no lag. I placed new renditions of fairy style shops for the Ambitions and WA ones on your small lots individually in a little group and still no lag. So one can have all those things on one world grouped separate but together, just not on the same lot. It was interesting to say the least - and I hope to see if I can fix the Glendale version. As lovely as that lot is - I fear it has a serious conflict and I must say the first single lot i have ever seen that bogs down an entire world. I tell folks not to put the ambitions lots with any other lots - they need their own little lot- oh well - it would have been nice had they worked together though. You ever get a chance check out that Fairy Tale Village lot by Elle - it's amazing to see anyway.
I've also gotten ML ready for the SN invasion just about. I have been playing different sims and getting up various useful skills I hope in between these other three worlds. ML works flawlessly for me anyway.