FSX was programmed for a different era. But there are a lot of tweaks you can do.
For example, check out this video. It helped me with a lot (but not all, unfortunately) memory errors. And to give you an idea of add-ons I have installed: my FSX folder is 266 GB plus I have a separate add-ons folder that is almost 43 GB.
There is a theory which states that if ever anyone discovers exactly what the universe is for it will instantly disappear and be replaced by something even more inexplicable. There is another theory which states that this has already happened.