File Caching
If a program reads data from a file, the OS will first copy (part of) that file into the system (file) cache (page cache on Linux). If the same or another program accesses the same data later, it will read it from the system cache and there is no need to read it from disk. (The left-hand side of the figure does not completely correspond to what happens when accessing TB data. The data is not copied from the system cache into the address space of the process, but the relevant memory pages are directly mapped into the address space.)
Copying files that are often used to a RAM disk will have the effect that the data will be in both the system cache and the RAM disk. It will be present in RAM twice.
Even if the system has lots of RAM, it will now have less RAM left for the system cache. That means that less of the other data and programs that are accessed often can be cached in the system cache.
400 MB on 32 GB total might not be noticeable, but it is clear that the impact will be significant when a RAM disk is used to cache a couple of GB of 6-piece TBs. That means a couple of GBs less for the system cache, which is already under heavy pressure when accessing big portions of the 68 GB of 6-piece WDL data.
Linux has a RAM disk implementation that somehow makes the content of the RAM disk be part of the system cache. That solves the problem of storing the same data twice, but still assumes the user knows better which files are good to cache than the system. I don't know if there is a RAM disk implementation for Windows that is as "smart" as that Linux RAM disk, but I doubt it. Probably only Microsoft could implement one.
In any event, on Linux one can simply do "cat *.rtbw >/dev/null" once and all WDL files are in the system cache (page cache) until the system determines that there is more important data to cache. I don't know if there is an easy way to achieve the same on Windows, but it would be simple to write a small program that sequentially reads all TB data once. Running that program once before launching an engine for the first time after boot would eliminate any initial slowdown that might occur when the 3-4-5-piece TB files are being cached into RAM during probing.
