To obtain further information on about the "HL3U bkgrd process", assuming another poster doesn't provide it, might require TAC. Or, the "Output Interpreter" tool might shed some additional light on it.
Something you might try, moving to a more recent IOS. Either 12.2(25)SEE4 or another later/higher SE release.
With regard to the capacity difference between many routers and hardware based switches, the former (i.e. software based routers, like the ISRs) rely on the main CPU. The latter, like the 3750, have ASICs (application specific integrated circuit), i.e. dedicated hardware, for normal packet forwarding.
As an example, the 7200's NSE-G2 (very high end for a software router) is rated at 2 Mpps, while the 3750 series tops out at about 38.7 Mpps. The former will show 100% busy CPU, but unsure what the latter will show for its CPU when totally saturated. (It might require a different show command, such as show controller[?], to reveal how busy the 3750 hardware is.)
One thing to note, both routers and switches have been optimized for their primary function but some types of traffic processed by either, especially switch traffic not processed by ASICs, will very much impact the device's CPU.
