Because right from the beginning it was designed to read in ALL the data from your entire game.

• Each time you get a new EP, that is over 100Mb more data it needs to read.
• Every time you download some CC that is more for it to read and work with.

To speed up SimPE and to reduce its size as far as possible, go into Extra/Preferences and the Filetable tab.

Untick all the EPs except your latest EP or SP and the basegame.  Make sure you don't also have a path to your Downloads.

For working with custom content, make a special "workshop" folder, and move the packages you want to work with or from into there temporarily.  Add just this one folder to your filetable path list.

If you cannot load SimPE in the first place in order to set up the suggestions in the previous post, then run it with the -localmode commandline argument the first time.  After you have set it up, it should be ok after that.

If you are not comfortable with the commandline way of running applications, create a shortcut from  SimPE.exe, then right-click it and choose Properties.  On the Target line, and after the quotes are closed, add space then -localmode

Run it from that shortcut the first time.