I bought version 5 today and had the same issue with the slowdown and lag in the IDE. I've found a tweak that works for me. Results may vary depending on your system specs. I have a AMD 2500+ with 768Mb RAM. Been running like this for a few hours now with no lag.
Here's what I did:
In the \Program Files\Zend\ZendStudioClient-5.0.0\bin dir, there lives a file named "ZDE.lax". From this file the executable launcher reads it's configuration data from. The key to the tweak is editing the options that set the initial and max heap sizes.
First, close the IDE and backup that file. Next, open the file in notepad find the two lines:
lax.nl.java.option.java.heap.size.initial=16777216
lax.nl.java.option.java.heap.size.max=201326592
and change the values to:
lax.nl.java.option.java.heap.size.initial=64777216
lax.nl.java.option.java.heap.size.max=256326592
Save and close the file and fire up the ZDE and watch it fly. I hope it works as well for everyone as it has for me!
-a9