another idea to check whether it have somewhat to do with windows memory managment might be:
you can, before you start to play MSFS empty the Standby Memory.
You can do this with help of RamMap, a sysinternals tool:
Example:
Empty → StandBy List
You can also first empty the WORKING Sets ( you will see much memory goes into the Standby list too ) and then clean the Standby list.
Then you have clean/fresh memory ready for MSFS 
