how to make Application work fast in low hardware config?

The perception of speed is sometime misleading.
Are you sure the problem is memory.
I have seen people spending a lot of time and effort speeding up part of the program which were not the really problematic.
You have to concentrate on the bottle necks.
In term of speed accessing memory is 1ms, accessing a file is 10ms, accessing the internet is 1 second.
Using less memory is an option.
Buying more memory is another. If you have a little bit more memory your hard disk swap less, and your pc last longer. This might be a time saver as much as a money saver.

If you're sure the memory is the problem then you can make your program use less.
Reference only the dll's you need. Use less libraries. Don't hold to much data in memory.