I have also been very concerned about performance of AVs. That's why I have used Clam Sentinel for years, which is an open source way of running the free AV ClamWin resident.
ClamWin is usually used mainly on servers but with Clam Sentinel, it is usable on individual PCs.
Its idle memory usage is better than the winner. When scanning it is about middle of the pack. Easy to disable if you just move files around on your computers and want to do it as fast as possible.
An AV that gives you a lot of control.
As a second opinion for downloads I use Emsisoft Emergency Kit's terminal scanner (formerly called A2Squared) triggered by Firesoft after a download has finished. I use the following batch file for that:
[copy or print]
@echo off
echo.
::After 3 sec default to n (No)
choice /T 3 /D n /M "Do you want A2 to do a RAM memory scan too? Press"
:: pressed N
if errorlevel 2 goto NOMEM
::Check for wrong keypress and assume no
if not errorlevel 1 goto NOMEM
:: pressed y (Yes)
c:\Apps\tc7\Msc\A2SquaredPort\a2cmd.exe /m /h /a /n /f=%1
goto NEXT
:NOMEM
c:\Apps\tc7\Msc\A2SquaredPort\a2cmd.exe /h /a /n /f=%1
:NEXT
@echo off
if not errorlevel 1 goto END
::Foreground yellow, type in red for better signal visibility
color EC
echo VIRUS ALERT: %1
echo.
pause
:END
echo End of batch program
exit
@echo off
echo.
::After 3 sec default to n (No)
choice /T 3 /D n /M "Do you want A2 to do a RAM memory scan too? Press"
:: pressed N
if errorlevel 2 goto NOMEM
::Check for wrong keypress and assume no
if not errorlevel 1 goto NOMEM
:: pressed y (Yes)
c:\Apps\tc7\Msc\A2SquaredPort\a2cmd.exe /m /h /a /n /f=%1
goto NEXT
:NOMEM
c:\Apps\tc7\Msc\A2SquaredPort\a2cmd.exe /h /a /n /f=%1
:NEXT
@echo off
if not errorlevel 1 goto END
::Foreground yellow, type in red for better signal visibility
color EC
echo VIRUS ALERT: %1
echo.
pause
:END
echo End of batch program
exit




.
) because of extension compatibility issues.
.