This is a build (built by me) without kdenext. (all 4 of my machines are running this build).
I don't have a massive memory leak with plasma-workspace
Now, I myself, cannot blame kdenext, as none of my 4 boxes are using it.
I also highly doubt it has anything to do with kdenext. it can be a magnitude of multiple problem(s) of packages, configs, removed /packages/libs, etc.
System: Host: x1 Kernel: 4.1.6-towo.1-siduction-amd64 x86_64 (64 bit) Desktop: KDE Plasma 5.4.1
Distro: siduction 15.1.0 Thunderstruck - xorg - (201509110914)
Machine: Mobo: Gigabyte model: GA-970A-UD3 v: x.x Bios: Award v: F7 date: 10/22/2012
CPU: Hexa core AMD FX-6300 Six-Core (-MCP-) cache: 12288 KB
clock speeds: max: 3500 MHz 1: 1400 MHz 2: 1400 MHz 3: 3000 MHz 4: 1400 MHz 5: 1400 MHz 6: 1400 MHz
Graphics: Card: NVIDIA GF106 [GeForce GTS 450]
Display Server: X.Org 1.17.2 driver: nvidia Resolution: 1920x1200@59.95hz
GLX Renderer: GeForce GTS 450/PCIe/SSE2 GLX Version: 4.5.0 NVIDIA 352.21
Audio: Card-1 NVIDIA GF106 High Definition Audio Controller driver: snd_hda_intel
Card-2 Advanced Micro Devices [AMD/ATI] SBx00 Azalia (Intel HDA) driver: snd_hda_intel
Sound: Advanced Linux Sound Architecture v: k4.1.6-towo.1-siduction-amd64
Network: Card: Realtek RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller driver: r8169
IF: enp4s0 state: up speed: 1000 Mbps duplex: full mac: 90:2b:34:a5:99:2d
Drives: HDD Total Size: 8001.6GB (24.9% used) ID-1: /dev/sda model: Hitachi_HDS72202 size: 2000.4GB
ID-2: /dev/sdb model: ST3000DM001 size: 3000.6GB ID-3: /dev/sdc model: ST3000DM001 size: 3000.6GB
Partition: ID-1: / size: 199G used: 9.9G (6%) fs: ext4 dev: /dev/sda3
ID-2: swap-1 size: 4.29GB used: 0.00GB (0%) fs: swap dev: /dev/sda1
Sensors: System Temperatures: cpu: 30.0C mobo: 18.8C gpu: 35C
Fan Speeds (in rpm): cpu: 2129 fan-2: 0 fan-3: 0 fan-5: 0
Info: Processes: 212 Uptime: 2:05 Memory: 1521.0/7971.5MB Client: Shell (bash) inxi: 2.2.28
System: Host: x1 Kernel: 4.1.6-towo.1-siduction-amd64 x86_64 (64 bit gcc: 5.2.1)
Desktop: KDE Plasma 5.4.1 dm: sddm,sddm Distro: siduction 15.1.0 Thunderstruck - xorg - (201509110914)
Repos: Active apt sources in file: /etc/apt/sources.list.d/debian.list
deb http://httpredir.debian.org/debian/ unstable main contrib non-free
Active apt sources in file: /etc/apt/sources.list.d/misc.list
deb http://packages.siduction.org/extra experimental main contrib non-free
deb http://packages.siduction.org/user experimental main contrib non-free
deb http://packages.siduction.org/user unstable main
Active apt sources in file: /etc/apt/sources.list.d/multimedia.list
deb http://www.deb-multimedia.org sid main non-free
Active apt sources in file: /etc/apt/sources.list.d/siduction.list
deb http://liquorix.net/siduction/base unstable main
deb http://liquorix.net/siduction/extra unstable main
deb http://liquorix.net/siduction/fixes unstable main contrib non-free