Windows 7, Java Version 1.7.0_45
When running this exact command:
/summon Item ~ ~ ~
The frame rate of my game goes to 0.
The Item SHOULD NOT BE PICKED UP, but must be left on the ground for the lag to occur. You need to change the ~ ~ ~ to some other nearby coordinates in order for the lag to happen.
Note: I had my settings to make my frame rate the highest possible, in a flatworld, and my frame rate dropped from around 180 to 0.
to the end of the command does not cause any lag.
(I recommend flying and running "/summon Item ~ ~-2 ~" in order to cause the lag)
Looking at some of the other bug reports, I assume it is just a problem with filling in tags that are not present.