I had not noticed that, but today I watched two of my older systems after I logged in, and on one I did see the black screen and cursor for a few seconds before the panel and then the wallpaper appeared. For example, here is an older laptop, running the nouveau video driver, that does not show your black screen at all:
System: Host: e6500 Kernel: 3.10-9.towo-siduction-amd64 x86_64 (64 bit, gcc: 4.8.1)
Desktop: KDE 4.11.00 (Qt 4.8.5) Distro: siduction 13.1.0 Firestarter - kde - (201305211844)
Machine: System: Dell product: Latitude E6500 serial: 7XFD1J1
Mobo: Dell model: 0PP476 serial: .7XFD1J1.CN129619122714. Bios: Dell version: A14 date: 07/31/2009
CPU: Dual core Intel Core2 Duo CPU P8600 (-MCP-) cache: 3072 KB flags: (lm nx sse sse2 sse3 sse4_1 ssse3 vmx) bmips: 9575.84
Clock Speeds: 1: 800.00 MHz 2: 2401.00 MHz
Graphics: Card: NVIDIA G98M [Quadro NVS 160M] bus-ID: 01:00.0
X.org: 1.12.4 drivers: nouveau (unloaded: fbdev,vesa) tty size: 141x38 Advanced Data: N/A for root
Network: Card-1: Broadcom BCM4312 802.11b/g LP-PHY driver: b43-pci-bridge bus-ID: 0c:00.0
IF: wlan0 state: up mac: 00:23:4e:ab:86:7a
Card-2: Intel 82567LM Gigabit Network Connection driver: e1000e ver: 2.3.2-k port: efe0 bus-ID: 00:19.0
IF: eth0 state: down mac: 00:21:70:d4:d3:04
Drives: HDD Total Size: 120.0GB (41.2% used) 1: model: OCZ_VERTEX
Info: Processes: 160 Uptime: 11 min Memory: 575.5/3948.5MB Runlevel: 5 Gcc sys: 4.8.1
Client: Shell (bash 4.2.45) inxi: 1.9.14
I would guess that your system is very busy doing something immediately after you log in.
-- do you have a lot in ~/.kde/Autostart?
-- under ~/.config?
-- under /etc/rc.local?
Or, if your GPU is an older model with limitations, maybe the new kwin is pushing it harder than it can handle?