Touch wood, looks like this problem is not only fixed, but improved.
Previously in normal operation a group of adverts called in one way were taking a sluggish 2.5s each to respond, whilst another, simpler group averaged about 0.2s, which is reasonable. ‘Normal’ wasn’t brilliant, and went intermittently haywire causing this topic to be opened and Jason to report the fault.
Now all adverts are called in the same way and all are responding consistently in the 0.2s region (good). Early days, but no long delays so far.

The zigzagging timelines are normal variations caused by slight server, network and client delays.
Dave