Although I've not seen any playback issues - the ITVX app on SkyQ is very very slow compared to any other device.
If you tried the usually "powr off for at least ten seconds" than a lagging would normally point to an Internet or Wifi issue.
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SkyQ Silver bundle (V2 2TB with UHD/HDR + two minis) in Sky region #71 (Oxford) using VirginMedia Gig1 Fibre (1.2Gbps/100Mbps).
