Hi Keith - welcome to the fleet!
I'll assume that you have the same installation as me, i.e. Yamaha 9.9 engine with manual lift, if not then my suggestions may not help. As I indicated in my reply to Jonathan I've found that the best arrangement for the cables is to get them as free as possible. I think the yard like to have things neat and tidy when they build the boats so they tend to add clips and cable ties etc to keep everything ship shape. Unfortunately I don't think they sail the 26 much so they don't realise that this doesn't actually help when using the engine. If the throttle and gear lever cables are tied together then it restricts the movement of one or the other when you try to turn the engine left or right, and this has an effect on the throttle position, and in my case I got the opposite effect to you, so when the helm was hard over to port the revs would increase, not a lot but definitely noticeable. I also found that there was a significant drag on the helm when the engine was engaged. I've cured both problems by removing all attachments to the cables, and also by making sure that when you drop the engine then you need to pull the cables up from below the steering rods, and you should now find that the engine revs will stay constant when turning and the helm drag is minimised.
As it happens I've just made a further improvement due to having the gear cable break. Looking for new replacement cables I found these from Force4 (and elsewhere of course)
https://www.force4.co.uk/item/Multiflex/Edge-33C-Control-Cable/EQO (you need 8ft cables, not shown on the web page today because I just bought them!) Their main attraction for me was that they specify a minimum bend radius of only 100mm whereas the Yamaha cables have a minimum radius of 200mm, and they are noticeably more flexible. I mentioned them to Matt at the Boat Show this week and he wasn't aware of them and showed some interest. I've only just fitted them and haven't had the boat out with them yet, but first impression is that I can hardly tell the difference in helm resistance between having the engine engaged with the tiller or free, so looking forward to trying it out for real. (When replacing the cables I also found another place where they had been tied, which was a cable tie holding them to the coil of surplus fuel hose lying on the floor.)
If you need to adjust the idle speed of the engine then the best way is to wind the plastic cable attachment to the throttle in or out a turn or two. Take the cover off the engine and you will see the throttle cable attached at the lower left side of the engine (looking back at the engine). Pull the small R-pin out and the end fitting will slide off the peg on the throttle lever to the left. Slacken off the lock-nut behind the end fitting and then turn the end fitting to make it either longer for more revs or shorter for fewer revs. Slip the fitting back on the throttle peg and test the new position by running the engine and engaging forward and reverse a few times to let the new position settle down. (One of the hazards to watch out for is setting the idle revs too slow, which can cause the engine to stall when going from neutral into gear - can be annoying when you need reverse in a hurry!). Don't forget to put the R-pin back, assuming you haven't dropped it in the river of course.
Obviously you won't want to be shopping for new control cables for a brand new boat, but do get the cutters and the screwdriver out and get rid of all the cable ties and clips, and you should find things improve.
Nick