I am playing Deus Ex again on my Dell Inspiron 15 and I notice it runs slowly especially when there is a lot of stuff occurring at the same time in the game.

Thing is, I remember playing it on much older computers and having it perform better. For example I remember it running perfectly on a laptop back in 2005. It can't be a hardware reason that it's running slowly.

Has anyone noticed the same thing? Has anyone found a workaround or solution?

My friend had a similar problem when he tried running DX on his computer. The issue is probably that your laptop's processor, by default, is splitting up the work of running the game between its cores. When DX was first made, it was optimized to run on a single-core processor. I can't remember what the fix is right now, but it should probably be easier to find now that you know the problem.

You can try using command line parameter -cpuspeed=xxx where xxx is the speed of your cpu.
If it fails, you can try disabling the extra CPU cores and leaving only one enabled.

This worked for me a while back.