Although an antivirus program or software basically is designed to protect a PC from Virus or Trojan horse however most of the antivirus programs available in the market also work as an anti Malware and also remove Adware and Spyware programs.
There are so many antivirus programs are available in the market and few of them are really working well, however there are few disadvantages with every antivirus program and that is it impairs PC performance and in most of the cases a computer becomes slow.
If we talk about a standard antivirus program or any computer Cleaner it should not affect PC performance /speed in any way rather than it should be protecting and after all increasing performance somehow.
Whenever we talk about to increase PC's performance it also comes in mind immediately that which antivirus program or Computer Cleaner software is using and if this is a heavy and occupy a large space in your PC one can also think to eliminate it from the PC.
I think using any antivirus or computer cleaner program in any PC is an important security aspect and necessary essential from PC's performance point of view.
Many times it is also seen that even after removing any computer cleaner or antivirus program there left some remains of the uninstalled software and these remains also create an issue like interrupting normal functionality of your PC and slow PC issue, freezing, shutting down etc.
This is a common case with not only for just an antivirus or anti Malware program but with all the Computer Cleaners available in the market.
I have also seen that few of the antivirus programs that are available online (both freeware and paid versions) are sometime very difficult while we uninstall them and they left more or less some of the entries in the PC.
Usually almost all the antivirus or Computer Cleaner programs follow signature based detection (for searching) within executable code (A virus is always an executive file), still there are chances where there is no signature i.
a virus can be found without this pattern.
In this situation usually antivirus programs follow a heuristic approach (Heuristic method or approach are used to speeding up the process to find better solution, where there is no other alternate or exhaustive search is impractical).
I would also like to mention another method that is also used by many Antivirus and computer Cleaner programs - File emulation which is in fact the same heuristic method.
However I won't discuss much about it here and now.
In the end while summing it up I would say that it is really not possible ever to develop such kind of protective program or antivirus or Computer Cleaner that can cure 100% without affecting PCs performance or without leaving any disadvantages in your PC.
However one needs to remember and follow a simple approach that an antivirus or computer cleaner or registry cleaner program should work in a way so that they do not slow down PC performance and also it should not impair any other application's normal functionality in any way.