Always nice that you are so public with your learnings, a good attitude to have. On performance tuning, although not being in day-to-day Domino development anymore, I remember a few years back reading the redbook "Performance considerations for Domino applications", if I remember that title correctly.
It is pretty much the only red book I ever read cover to cover, and takes a really deep dive into Domino, its internal workings, and the costs of doing certain operations.
If you have the time and the interest, I recommend it. If you don't, the summary is "VIEWS". Almost all performance problems originate there. Indexing, #of docs in a view, use of readers fields, #of columns, # of sorted columns, column formulas, date calculations...there is a long list of design considerations that will make your view shine or fail. I advise not to waste time millisec-tuning a Lotusscript string concatenation. Focus on views and you will win the most.