preloadcamera precatches an area around the indicated position for immediate usage by real active camera. The preloading might take more or less time and more or less memory depending on detail level, vcard ram, etc. So, just after executing that command the area is not preloaded, you still need to wait a bit.
for SQS
@!preloadCamera _pos
for SQF
waitUntil preloadCamera _pos
If you execute several preload operation one after another, more than probably you will run out of cache memory quite fast, so after your last preload command, more than probably the cache of first preload has been already lost.
If you have a cutscene, preload first the area of that cutscene, and while the cutscene is running, preload the area of the next one and so on.