I have 32GB 3200.
With 16 GB your system starts to page memory to disk.
Conrtrol your paging file settings. Where is your page file located?
On HDD it is very slow, On SDD the speed is ok but it does not replace ram.
Paging should always be avoided.
Increase our memory or
Check your settings in MSFS if nessesery reduce some setting which are responsible for memory consumption. I think these are mainly the 5 settings under Anti Aliasing .