The easiest way is to output the slideshow as a video, then load this video to your timeline, right-click and apply Time Stretch to speed it up. To turn 5 second slides into 1.5 second slides, you'll want to set the speed to 3.57x.
To output a standard definition video, use Publish & Share/Computer/AVI with the DV preset.
To output a high-definition video, use Publish & Share/Computer/AVCHD with the M2T 1920x1080i preset.
Premiere Elements 11 presumed on Windows 7, 8, 8.1, or 10 64 bit
and assuming that all the photos whose duration are to be reduced have the same initial duration.....
Based on the thread title....
1. Select all the photos involved all at one time. To do that, hold down the Shift key and click on each of the photos whose duration you want to reduce.
2. Right click anywhere in the selection, select Time Stretch from the resulting pop up menu.
3. In the Time Stretch dialog, change the duration reading.....if photos are now 5 seconds (00;00;05;00) and you want each to be 1.5 seconds, then set the Time Stretch duration for 1.5 seconds (00;00;01;15)....hours;minutes;seconds;frames...if you are in a NTSC setup (30 frames per second)...15 frames = 0.5 second.
Please consider and let me know if I have targeted what you sought.
Add On...With regard to the Time Stretch dialog Speed % setting...there is no actual set speed to 3.57x. The actual setting is in percentage. More information on that although I do not see this involved if I have interpreted your question correctly.
I'd hope we all remember our high school math enough to know to transpose 3.57x to 357%.