I am at a partial disagreement, I use teh latest stock kernel on all my machines from a p2 366mhz (my p2 233mhz machine died during a bios update), a celeron 400 , a p3 700 , a couple of older centrino's, a dual core centrino two semprons , and thats just teh laptops.
Once on the machine it runs just fine, certain hardware needs more work or work arounds, bu the kernel itself is not the "heavy" part,
the "stock" services are more likely to be overloading available resources , which is why I only have minimal start up seervices on te -light range of iso's
with lighter machines with less resources , you do have to "cherry pic" your apps, in its last revision the midori devs removed some of its "legacy" code (which made it easier for older hardware) which is why that app probably started to misbehave. xxxterm in the repo atm is buggy with plugins but works ok (I actually use a self compiled version 1.81 which fixes that issue), also Surf is a capable light browser (at a cost to some functions you may like - although new versions of surf with various patches to make it full featured exist - on the suckless.org site)
another area that can eat up resources is window manager / DE, try out a light window like icewm and see if you suffer the same issues ( I use ratpoison atm as a personal choice)
You can also adjust the 'swappiness' factor to make your swap partition more available
Its easy just to blame one part of the set up for issues like you describe, but more often than not its a mix of things happening at the same time (they call it "progress") that stifles older hardware.