As Rolf indicated, it probably has something to do with the designed shape of the spinnaker more than how you are trimming it at this point. However, I do see a couple of things that might help.

1) as others have pointed out, getting the tack to stay put is important so you keep the same amount of luff tension that you intended on the beach. As the tack line slacks off, the sail will rotate to leeward and the sweet spot for trimming it properly gets narrower making it harder to keep trimmed when you sail closer to the correct angle. You will probably find that you can sail it higher and hotter if you can maintain the correct luff tension.

2) in the last segment of your video, your jib is over trimmed and we can see the leeward tale stalled almost 100% of the time. Pay attention to the jib trim even downwind - dirty air coming off the jib is dirty or overly compressed air over the main (meaning that it's hard for it to stay correctly powered from top to bottom) and you're losing some power there. It's much better to have the jib undertrimmed than overtrimmed in this scenario.

3) Upwind (assuming you were trying to sail upwind there), the mainsheet still doesn't appear to be sheeted enough. There may be some twist in the camera lens but I'm still seeing a good bit of the upper mainsail from that angle and the jib headstay looks like it's sagging a bit. Additionally, I see that you are having to foot off to stay powered up - the leeward jib tale is stalling occasionally...Stalling the leeward jib tale is worse than pinching. You may be going fast but you are giving up too much ground. Conditions don't look to terribly windy there, have the crew sheet in until he can't anymore and see what the boat feels like. If you are too powered up, you need to start depowering with downhaul and mast rake and lifting the boards a little can help settle the boat down. You can actually reach a point that if you keep sheeting harder the mainsail starts to depower. It's uncomfortable at first but once you experience it, you will quickly start to feel the efficiency. I remember the first time I had some coaching (Robbie Daniels) and he was behind me yelling "Sheet In" repeatedly through a bull horn. I thought I was really good upwind and I couldn't believe how hard he was telling me to sheet (certain my boat would break). When he finally said "there" I had about everything in it I could muster and the boat was on a rail and absolutely flying upwind. I'll never forget that sensation and it is the same on my F18 (that was on my a cat).

Setting the luff cord on the spinnaker is not super critical but should be checked (we check ours once per regatta). Basically, untie the luff cord at the tack so that it is loose (don't lose it!). Hoist the spinnaker and set it to your normal luff tension. Mark the luff cord where it exits the sail with a sharpie. Pull the luff cord 25mm (1 inch) and tie it there. You just want the luff cord to carry most of the luff tension without putting slack in the sail - it's good for the longevity of the spinnaker.