I have just this week moved to fibre (well VDSL), from ADSL. On ADSL I
was connecting at 18Mbs, speedtest at around fairly consistent 16Mbs.

I was moved in the early hours of Tuesday morning, since when speedtest
results have been all over the place - sometimes 31Mbs, sometimes
10Mbs. My initial tests showed a steady, 30Mbs with the ISP supplied
dumbed down router.

I bought a second identicle router, but with OpenWRT installed. I spent
much of Tuesday trying to gets it configuration settings to work,
swapping to that and back to the ISP router, between times, until I
finally got the OpenWRT to work - so multiple times.

Throughout, a connect speed was reported of a steady
40,000Mbs/9.000Mbs, yet actual speedtests showed wild variations. Are
the wild variations part of the system settling in and how long might
it go on for?