Exactly what is getting slow?
Your preview? If so, that may be normal since the program has to generate all the transitions and other effects and may not be able to keep to real time. Also, if you can make resized (720x480 is good for the USA) copies of your pictures and use those instead of the enourmous pictures most cameras produce, you may get a really good speed up.
Just moving the editing cursor on the timeliine? If you have several hundred pictures it will take longer since each picture has to be accessed as you scrub.
What sort of styling and transitions are you using between pictures? Each takes a different amount of computer time to calculate each frame. Rendering your slideshow may take a lot longer than the total length of the video. This is normal.
How is your windows setting for the hard drive and virtual memory set up? Are you letting windows manage the virtal memory? Sometimes you can get a speed up by defragging the harddrive then setting the virtual memory manually. Try a value of four times your installed ram. This should be plenty and windows won't have to thrash about looking for free drive space.
Have you got a large harddrive? If your remaining drive space (less than 20%) is small, wndows can get very slow. MEP will need lots of space to make temporary files, probably one for each picture, since your pictures may be huge.
Fav quote - "Experience is whatcha don't get 'till ya don't need it no more."
System - Athlon 1.4GHz, Win98, Hauppauge PVR250 receiver and compressor.
Software -Magix Movie Edit Pro 10, Nero 6 + NeroVision Express, Moho 4.61, PSP 8.1, Bryce, Quicktime 6.52 pro, Goldwave 5, DVD-Lab.
Cameras - Panasonic GS9, Canon ES8400V, Canon EOS D20 and Canon A70