840 Pro can't handle multiple large writes

.

Sure you're using AHCI and have TRIM enabled? Kinda sounds like what happens when you're in IDE mode or TRIM is disabled, and you're relying on the drive's builtin garbage collection. I don't seem to remember any critical firmware updates for the 840 Pros, but you could install Samsung Magician and see what it says. Don't let it do any of the OS optimisation changes, just the firmware/drive health stuff.

Check firmware like JBark said.

What's the rest of your system like?

Yeah check trim. or something else goin on. Some of my samsung ssds have over 200TB written used as cache drives and they still rockin strong.

This is quite common among most of the consumer grade SSD's.
On Servethehome, all consumer SSD's are marked as pile of shit, crap performance, worthless and so on.

On Ebay, enterprise SSD's are available for same $ per gb as consumer SSD's, but for example the Intel S3700 series still have 20x more life left.

I posted a thread earlier that show how crap consumer SSD's really are. The old Intel 320 is still faster than Samsung 960 pro for write intensive workloads. https://hardforum.com/threads/benchmarks-of-real-world-write-performance.1936669/