SUMMARY
During using Plasma 5+KWin on Nvidia GTX 660 and nonfree driver, extreme lag spikes happen. I will show few examples in attached video.
If I uninstall driver and take out the video card, it works fine on integrated graphics.
Also using nouveau is smooth, but it crashes my machine very often and only SysRq works. Also, I don't use that card to stare at desktop but also to play games (and that's quite impossible with nouveau).

Video: https://streamable.com/ti71n (attachment is too big)

STEPS TO REPRODUCE
1. Get GTX 660.
2. Install Debian 10 with KDE.
3. Install nonfree driver (nvidia-driver).
4. Try to use it for a day. If you can't see it, use KWin FPS meter applet.

OBSERVED RESULT
Extreme lag spikes.

EXPECTED RESULT
Smooth experience.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Debian 10 (stable)
KDE Plasma Version: 5.14.5
KDE Frameworks Version: 5.54.0
Qt Version: 5.11.3

ADDITIONAL INFORMATION
$ inxi -Fx
System:
  Host: pecet Kernel: 4.19.0-6-amd64 x86_64 bits: 64 compiler: gcc v: 8.3.0 
  Desktop: KDE Plasma 5.14.5 Distro: Debian GNU/Linux 10 (buster) 
Machine:
  Type: Desktop System: Dell product: OptiPlex 790 v: 01 
  serial: <root required> 
  Mobo: Dell model: 0J3C2F v: A00 serial: <root required> BIOS: Dell v: A07 
  date: 09/10/2011 
CPU:
  Topology: Quad Core model: Intel Core i5-2400 bits: 64 type: MCP 
  arch: Sandy Bridge rev: 7 L2 cache: 6144 KiB 
  flags: lm nx pae sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx bogomips: 24745 
  Speed: 2035 MHz min/max: 1600/3400 MHz Core speeds (MHz): 1: 1781 2: 1677 
  3: 1723 4: 1745 
Graphics:
  Device-1: NVIDIA GK106 [GeForce GTX 660] vendor: Micro-Star MSI 
  driver: nvidia v: 418.74 bus ID: 03:00.0 
  Display: x11 server: X.Org 1.20.4 driver: nvidia 
  resolution: 1280x1024~60Hz 
  OpenGL: renderer: GeForce GTX 660/PCIe/SSE2 v: 4.6.0 NVIDIA 418.74 
  direct render: Yes 
Audio:
  Device-1: Intel 6 Series/C200 Series Family High Definition Audio 
  vendor: Dell driver: snd_hda_intel v: kernel bus ID: 00:1b.0 
  Device-2: NVIDIA GK106 HDMI Audio vendor: Micro-Star MSI 
  driver: snd_hda_intel v: kernel bus ID: 03:00.1 
  Device-3: Logitech Webcam C270 type: USB driver: snd-usb-audio,uvcvideo 
  bus ID: 2-1.6:5 
  Sound Server: ALSA v: k4.19.0-6-amd64 
Network:
  Device-1: Intel 82579LM Gigabit Network vendor: Dell driver: e1000e 
  v: 3.2.6-k port: 4040 bus ID: 00:19.0 
  IF: eno1 state: up speed: 1000 Mbps duplex: full mac: 18:03:73:b0:66:be 
Drives:
  Local Storage: total: 1.05 TiB used: 240.44 GiB (22.4%) 
  ID-1: /dev/sda vendor: GOODRAM model: GOODRAM size: 111.79 GiB 
  ID-2: /dev/sdb vendor: Seagate model: ST1000DM003-1SB10C size: 931.51 GiB 
  ID-3: /dev/sdc type: USB vendor: Kingston model: DataTraveler 3.0 
  size: 29.08 GiB 
Partition:
  ID-1: / size: 58.56 GiB used: 35.53 GiB (60.7%) fs: ext4 dev: /dev/sda5 
  ID-2: /home size: 915.89 GiB used: 204.91 GiB (22.4%) fs: ext4 
  dev: /dev/sdb1 
Sensors:
  System Temperatures: cpu: 47.0 C mobo: N/A gpu: nvidia temp: 53 C 
  Fan Speeds (RPM): N/A gpu: nvidia fan: 38% 
Info:
  Processes: 224 Uptime: 35m Memory: 7.75 GiB used: 2.82 GiB (36.4%) 
  Init: systemd runlevel: 5 Compilers: gcc: 8.3.0 Shell: zsh v: 5.7.1 
  inxi: 3.0.32