<p>Since some time, my Delphi debugger became much slower than I was used to before.</p>

<p>I noticed this in both Delphi 2007 and 2009, so it seems it's something outside of Delphi itself...</p>

<p>What causes this behaviour, and how can I prevent this?</p>

After weeks of searching for an answer, I found this Microsoft knowledge base article fixes my problem :
http://support.microsoft.com/kb/321410/nl
The point is, I installed Microsoft Script Editor a while back to do some server-side javascript debugging. This also installs a service called "Machine Debug Manager" (Mdm.exe), which is the cause for the slowdown. After disabling this service, debugger speed returned to normal!