I too have been suffering from painfully slow LR4 speed. I had been using a 5 year old Intel Q6600 2.4, 8gb, ssd system and it was fine for LR3.
I just upgraded to a home-built i7-3770k 3.5ghz, 16 gb ram, ssd and LR4 is blazing fast. There is no wait switching between modules. Preview render times are under 5 seconds, compared to 30 seconds before.
So I agree, if LR4 seems slow, you need a new processor.
If anyone is interested, here are the specs on my new system
Intel i7-3770k cpu
Asus P8Z77-vpro motherboard
Coolermaster 212+ cpu cooler
G.skill ripjaw 2x8gb ram
Samsung 830 256gb ssd
cost - 957$
ATI Radeon 4870 video card
Coolermaster Cosmos 1000 case
Corsair TX850w psu
cost - about $300, these were reused from my previous system