if there are a lot of images in the screenshot directory it can delay things -- maybe try moving some of your screenshots into subfolders every once in a while.. i've been meaning to improve the thumbnail speed for a while now and it's still something i plan to do.
Regarding your clipboard text naming.. its not a bad idea for a feature.
Once thing you can do which might be good enough for you is that you can paste the clipboard text in the name field of the screenshot and hit enter, and that will rename the image file. That's pretty fast.
If you really needed to do a lot of captures where you always used clipboard text.. an easier thing for me to do might be to add such a function to MiniCap