In my case I am almost certain that it is down to the size of my community folder.
I run Ccleaner a few times a week, check for malware two to three times a week and defrag at least once.
I run W10 pro 64bit on an i5-6500@3.2 ghz, with 16GB RAM and a Nvidea GTX1060 6GB.
MSFS is installed on a 7200rpm 4TB HDD.
I occasionally start with a default game, which loads noticeably faster, just for a world or Sim update, although WU5 and 6 I forgot to rename the community folder until it was too late and the game was already updating.
I don’t use the addon linker because, pardon the pun, I am flighty. I look at the map choose a start and finish, I used to choose waypoints too but I can’t seem to do that at the moment, then I click fly. I rarely start MSFS with specific intention but instead, I choose routes at random, thus I need it all installed before it starts.
I look for places I have been, or see in documentaries and it gives me a new perspective on the world, a birds eye perspective if you will.
So keep it coming Asobo, I patiently await WU8 and all the treasures it will bring 
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