It is generally true that the fuller a drive becomes, the slower it operates. With hard disk drives this is due to diminished sectors per track on tracks nearer the center of the drive platter and a greater distance required for read-write head traversal.

So if you have a 4TB disk drive and only use the outer 1cm of the platters, your drive will stay quite responsive. To avoid wasting drive space, it makes sense to partition additional logical dives on the slower, less efficient inner part of the drive platters for large contiguous files that do not benefit from extremely fast random access latency—like movie and music files (a song takes three minutes to play whether it loads to RAM in a microsecond or a hundred milliseconds)

System memory uses a very optimized memory hierarchy which trades speed for capacity. This might be a typical high-end consumer grade PC memory model:

L1 Cache: 64K -- extremely fast instruction/data on each core 
L2 Cache: 1MB -- larger, but slower SRAM near each core 
L3 Cache: 6MB -- on-die storage shared by all CPU cores 
System RAM: 8GB -- off-die storage for all active processes 
Persistent Storage: 
NVMe SSD: 250GB -- Extremely fast file storage (most expensive) 
SATA SSD: 500GB -- Half as fast as NVMe (less epensive) 
SATA HDD: 1-2TB -- High speed like WD Black (7200-10000 rpm) 
SATA HDD: 3-8TB -- Lower speed, high capacity (5400-7200 rpm) 

My recommendation for the fastest performance gaming/theater PC for the money would be a three drive design with a 250GB Samsung 960 EVO NVMe M.2 boot drive, a 500GB 850 EVO SATA SSD for game installation, and a 4TB Seagate Barracuda (w/256MB cache) partitioned as four logical drives with the first TB for apps, and the other three for your media storage and a downloads partition. Each of these drives will cost you about $120 and give you impressive levels of speed for years to come, as long as you are judicious about where you save things.