I don't know if this help, but I have experience some of this at times.
I have found that the "quality" option if not set to 100 makes changes to jpg. Sometimes I forget to uncheck or check the "aspect ratio" when changing resize or canvas size, then I do a bunch of work not realizing this (face palm).
When I get a huge lag doing a project, I realize that my computer has a very small brain (celeron D) and it becomes very slow, thus my patience is tested. SO, whatever I am doing it really uses up the CPU power and creates the lag.
Take a look at the tutorial on Polaroids, montage, also someone did a great tutorial on a filmstrip, another fav. Create the border you want, save it for future use.
Some one will come along here and give more tips that will make mine become irrelevant.
good luck
2:58 am that might something to think about too.