So, a partial fix; knew something must be staring me in the face.
The vectors don't seem to be the problem, AGCI might be slowing things down though when it scans for contacts, not sure, have to test more, but then you have to rely on the IL2 vectoring, or play god and use the map.
UPDATE; so I did test some more, and it is the vectors. Shit.
Watched the RAF flights closing in on a lone raider on the map, waiting, pause... and just after the RAF group splits and breaks off. This at 5:03 mission time, so start again, run up to nearly 5:03, watch the RAF flights, waiting, pause... and they get new orders and vector toward the target.
So, the number of vectors or the number of flights being vectored? Tried it with even more vectors so they each control fewer planes (I know it says all planes within range, but it does actually work). No help, which is odd, on the big map I didn't really have this problem.
Still, try it with a single vector, better, still pauses, but only momentarily.
One red and one blue? Makes things worse again.
So, looks like only one (and no it's not in the read me), so here's a new single and repositioned entry for the mission templates to replace the vector block at the top of [NStationary]
2719_Static vehicles.stationary.Vector$VectorUnit 1 120000.00 290000.00 360.00 0.0
i.e. replace the eight entries with this single one. There will still be a bit of lag occasionally, nothing is perfect, but should be playable.
Does mean the Luftwaffe have some gaps, so Ventnor has probably been hit again, and the RAF may tend to bunch up more, but if it works...
There may still be some pauses, like when delayed start planes get spawned, but much less.
ALSO:
The Fire objects from C&Cv2 increase the draw distance to make city fires visible over long distances at night (Semor's campaign?), but this also seems to drop the frame rate by half on my set up, which maybe stands to reason... I put these in experimentally at the last minute (for Blitz fires in London), but taking them out of the mission templates (and generating a new campaign) seems to improve things. Delete these from the mission templates (BoB7, BoB8a, etc.) to increase frame rates.
Dynamic weather causes a slight delay when the weather changes, graphic update, which may contribute too, but removing it doesn't seem to make that much difference, and the changes are a bit sudden, but it does give some nice mixed weather effects with Stormfront, which doesn't seem to be a problem either really, there are two red and one blue which are enabled/disabled by DCG as the front moves over them so you may get none or all three.
Doesn't seem to be any real conflict, but we probably don't actually need both as the stormfronts are variable thanks to DCG and actually provide more realistic transitions without the sudden changes, and different conditions in different places on the map, so you could take out dynamic weather and stick to the stormfronts to reduce the graphic updates and give less sudden changes, but it's personal taste there.
After editing you have to start a new campaign or go into the campaign itself in Missions/GB/DE and edit the DCG campaign files (if you know what you're doing!). Easier to just generate new, unless you insist...
You should do this after disabling in jsgme of course, or the changes won't stick.
(You can actually do this anytime in a campaign, disable in jsgme, untick third party campaign in DCG, play a different one, come back again by re-enabling third party campaign, and point DCG to the interrupted campaign with Files/Select DCG Campaign Folder, or just use the folder icon on the button bar. Your campaign will start up again where you left it... ).
I'd also like to say thanks to pierre and Semor for giving me some feedback and suggestions, really helps to bounce ideas around, cheers guys.