If it goes lower throughout, that isn't really that surprising in an RTS game, as more buildings and units are made, that is more stuff that has to be stored in memory, and more processing power used to cover each and every action done by each unit.
You can tell if it is a CPU or memory problem by right clicking your clock in the bottom right and clicking task manager. Go to the performance tab. Graphs of CPU and memory usage will begin to be made. Open and start playing SC for a bit, then either alt + tab out, or close the game. Check the graphs, if either are pegged at 100% (highest point on te graph for a long period), that is the cause of your problem.
I'm not sure if it will work with integrated laptop graphics, but you can tell if it is your graphics causing the problem by downloading MSI afterburner from here:
Open it, and this will create a graph of graphics usage. Again, go and play the ame for a bit, and see if at any point it is at 100% for a long period of time.
Whichever is stuck at 100%, that is your problem, it is at its highest performance, which is too low. If it is memory or CPU, it is possible to fix it by closing all programs that you don't need open before you start playing. This will free up memory and will make less processing power be used on programs that aren't your game.