KDE freezes for few seconds after login

This was happening to me for a while, with different KDE versions. KDE desktop is loading, almost before all startup programs are running but suddenly everything stopped. The plasma workspace freezed for 5-20 seconds. This was going on randomly, sometimes it has frozen, othertimes not.

There was an issue, which KDE could not handle, with my sound card. It had the same IRQ (interrupt) number as my network card, so it had to wait for my network to be initialized and then it could continue with sound initialization.
Maybe it will be enough for you to just disable Login sound. You can do this in System Settings > Application and System Notifications > Manage Notifications. Choose event source: KDE Workspace and disable Login sound.

If just disabling the sound doesn't help, you can try to adjust IRQ number of your sound card.
To view system IRQs run: cat /proc/interrupts

If there are two devices using the same IRQ, you could have a problem.
To change IRQ number create new or add to an existing <something>.conf (the .conf extension is mandatory) file under /etc/modprobe.d/ following: options snd-hda-intel enable_msi=1

Reboot and view cat /proc/interrupts again, the snd-hda-intel should be the only device using assigned IRQ number.

UPDATE: the issue came back after some time. The next option was, as it seemed to be a sound issue, to disable sound daemon. In KDE go to System Settings >  Startup and Shutdown > Service Manager and under Startup Services disable the KMixD. For now this freezing problem is gone.