Ok, I've narrowed down the cause of slowdown slightly. It is when my laptop is on battery only that load time takes about 25 seconds. When plugged in (mains power), load time is < 1sec.
The only change I see in journalctl -xef
when switching between battery/mains power is as follows (first line battery, second line mains):
May 22 23:45:35 rosie kernel: Dynamic Preempt: voluntary
May 22 23:45:54 rosie kernel: Dynamic Preempt: full
Edit: I can also simulate the effect via sudo tlp ac
(fast load time) and sudo tlp bat
(slow load time).
No, I haven't updated packages in 3 or 4 weeks.