Making lap joints takes time and effort. When the joints are at 90 degree angles the cuts are relatively simple. When the cuts are at other angles it’s difficult to not only maintain parallel edges to each lap, but to ensure that each piece nicely fits into the other. If you go very slowly with your cuts you can ensure that the pieces fit. But if you have a lot of them this slow technique is hard to maintain.
Make the cuts for the lap joints at the same time with a jig that guides the work to ensure that the edges are parallel and consistent. I made a jig that could be fitted over a couple of boards that would eventually be fitted in a lap joint. By keeping the boards parallel the angle of the cuts would perfectly the mated lap. The jig was designed and built to guide the circular saw to make perfectly parallel cuts. Because the jig guided the edges it’s possible to spend time aligning the jig, but make the cuts quickly and without doubt. Once the cuts were made I removed the jig, knocked out the waste material and fitted the laps together.