The newer IDE's have had the agressive cache feature for a while now.SO not sure what you are really trying to say unless you have some very old hardware.
Wouldn't it be much faster if the compiler could use a RAMdisk to do the stuff compilers do ?
Not with a modern operating system / file system.
Do "modern" OS/file systems read and write with the same speed on Ram disk and SSD ?
Yes. Write-behind cache is the key-phrase. Using a RAMdisk just steals memory from the operating system.
There used to be a preference for build directory location...I'll see if I can hunt it down...