please can anyone help me why opening any webpage overloads 2.79 Ghz CPU for 1-2 seconds?

I have old Intel, single core, single thread CPU, but still it appears to me strange it uses so much CPU, because when there was Intel 486, Pentium 1, Pentium 2 computers, it had like 40Mhz, 100Mhz, 200Mhz CPU and still webpage loaded quite OK. so why the *** it ovelroading almost 3ghz CPU core?!

It happens In IE, Firefox and Chrome. Quite no matter which page i load. RAM appears to be like 50% full. Tried Firefox safe mode and turned off antivirus/firewall, but no change.