AV clients can use up CPU time and slow down performance when both the AV client and your Operating System are tyring to use the same file. This is common for executables because while the OS is trying to load the exe, the AV client is trying to scan the file for malware. IT is common in scenarios where you may be running a service, say a SQL server, to exclude the SQL files from being scanned by the AV client.
Another example where you may want to exclude scanning is say you are running a virtual application such as Virtual PC or VMWare Workstation. You would not want to scan the VMs with your host AV client. In addition, even worse, you would not want to load an AV client inside of the VM guest. Now you would have an AV client on the host and an AV client in the guest all trying to scan the same files.
Most A/V products perform what is called "on access scanning" in that when you start a new program, then the A/V product will scan the program file and all loaded components (shared libraries, and other files) before allowing it to run. This can cause serious performance problems, as you have seen. Once they are scanned and loaded into memory to run, then the system load will return to normal. Let's say that the executable and its associated libraries and other data comprise 100MB, then the scanner has to read through and analyze all 100MB EACH AND EVERY TIME it is loaded into memory. Myself, I usually disable that "feature" since I scan stuff before I download or install it on my computer. I use ClamWin, an open source A/V program. It does not do on-access scanning, though it can scan memory as well as files on demand, and it can be integrated with your email and browsers to keep malware from getting into your system in the first place.
yea I know that it scans exes every time I load them them. So it takes longer for program to start. but the thing is that even after about 15mins,30 or 1 hour sometimes,this happens for about 1-3 minute. again after 30mins. and again....after an hour while.I noticed this most when playign the game.