Yes, transferring data files from your expansion drive into the computer or from the computer into the expansion drive can make your computer run slowly.
If it's music and/or movies you are transferring back and forth it's simply because the computer is using most of the processor speed and memory to do it. That's why when another program or the internet is opened your computer will appear to run slowly.
If it's just slow in recognising the contents of your expansion drive it may be due to the age of your computer.
Alternatively it may be that your computer and/or expansion drive is fragmented and has registry errors. Both of these will cause - can cause - your computer to appear slow to load.
You can defragment your hard drive from within the computer. Using Windows XP, open 'my computer' andright click on the 'C drive icon' icon and then choose TOOLS. Open the defragmenting software and let it defrag the hard drive.
To fix registry faults search for and download some free software called free Windows registry repair. http://download.cnet.com/Free-Window-Registry-Repair/3000-2086_4-10606555.html
When downloaded, run the registry fixer by opening the program and click on 'scan'. Wait until the scan is complete and click on 'fix'. Job done...