It took ages to solve idle issue, but finally cracked it for my XP.
Windows XP very slow after idle appears to be issue with pagefile system cache (PF Usage) taking up exessive I/O activity on the disk (writing to harddrives) due to highly fragmented pagefile.sys file. Hence no real CPU usage showing, and changing PF Usage levels may not have much effect as it is not the size of PF, it is the fragmentation of the PF (normal defragment tools do not touch this file).
There is no more slowdown after I did a defragment of pagefile.sys with a tool that can actually access it (PageDefrag v2.32 By Mark Russinovich). An even simpler solution may be to set computer to clear pagefile on shutdown (I didn’t try this but probably works too - see below).
The high I/O was not from a rogue program or virus. It was slow buildup in pagefile.sys file fragmentation, the disk area storing current virtual memory blocks. My pagefile.sys had something like 264,000 fragments. Virtual memory is stored in 4KB blocks, but the fragmented block pattern was taking up excessive I/O for the drive to read after computer had been in idle.
During idle lots of application data is sent to pagefile rather than kept in RAM. Then when you start using applications again the computer is getting it back out of pagefile.sys: but if the pagefile.sys is highly fragmented then it can be painfully slow disk read speed. i.e. I sit there with almost nothing happening for 30 seconds, or watching webpages load almost one pixel line at a time.
I defragmented pagefile.sys, but maybe it is simpler to clear the file, something like:
Click Control Panel
Click Administrative Tools
Click Local Security Policy
Click the "+" next to Local Policies
Click Security Options
Doubleclick "Shutdown: Clear Virtual Memory
Start Registry Editor (Regedt32.exe).
Change the data value of the ClearPageFileAtShutdown value in the following registry key to a value of 1:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management
If the value does not exist, add the following value:
Value Name: ClearPageFileAtShutdown
Value Type: REG_DWORD