discontinuing certain compilation optimisations and heartbleed
I could have sworn that parts of the slowdown occurred because they globally disabled some accelerated math functions that in particular have huge impact on SSL/TLS optimisations, as well as that of the chrome pepper flash player plugin. I think that removing the compilation switch for SSE and possibly SSE2 was one of them (or at least that I've noticed on an older machine) so a whole bunch of relatively slow machines (because they don't have AVX/SSE3(x)/SSE4(x)) have become extremely slow. The only problem is that I've tried to find confirmation on this but have not tracked down through either release notes, or bug tracker, and I can't remember where I read this.
It was also roughly at the same time of heartbleed and some client side SSL/TLS safety checks were included.
In some cases where I've deliberately crashed chrome, I have gone from a usable 100 tabs across 10-15 windows under Chrome 30 and re-opened in 60 secs to a completely unusable mess that requires closing multiple windows to get down to a total of 15 tabs capable of running in less than 5 minutes.
People who have AES-NI or similar will have next to no problem (if there is any will be un-noticable because of the operation speed) as the slow down on non-AES-NI capable machines. This CPU capabilities flag as well as that of maths accelerations should be noticeable in the problem reports - that is if they are collecting it. This may explain why some recent intel chips fail, whilst older ones work as stated in the story..
It should be relatively simple to determine, go to portableapps, get all versions from 30 onwards, load up several tabs each of facebook, gmail, twitter, tumblr, pinterest, etc amongst others with mind numbing amounts of ads and flash and crash it (kill chrome.exe from task manager it should be the process without any flags/switches if you have command line column active.) Restart and open chrome task manager (Shift-Esc). If you are really committed some debug/profile software may help.