I don't understand the meaningful of original clock speed.
intel i5 4250u has only 1.3mhz turboboost 2.6 15watt for macbook air 2013
intel i5 4350u has only 1.4mhz turboboost 2.9 15watt
intel i5 4258u has about 2.4mhz and max tb2.9 28watt
they have almost all the same detail except the latter has 5100 gpu , others has 5000 gpu.
Are there any drawback of having lower base clock speed in 4250u and 4350u?
Does it means 4350u comsume lower power at 15 watt at 1.3mhz but it can be as powerful as 4258u at turboboost at 2.9mhz on demand at 28watt?
If then why don't they do original clock speed at 0.1mhz which turboboost to 2.9mhz and only consume 1watt?
1) CPU's will Turbo at least one of their cores up to the Turbo frequency. I think the reason 1.4GHz was chosen, despite the 2.9GHz Turbo capability was for power efficiency in a THIN laptop, both to minimize heat and increase battery life.
2) The HD5100 graphics will consume slightly more power and provide slightly better performance but I'm simply guessing at how much so far as I can't find benchmarks (my rough guess is 50% as in 1.5x better than HD5000).
"TDP" means "Thermal Design Point". It does NOT mean the amount of CPU power used (though it can never use more than this). Several CPU's can fall under the same TDP number. It's a convenience for manufacturing OTHER parts such as motherboards and CPU heatsinks. If Intel says they have a 15W TDP then someone like Asus knows what heatsink to use and fan to move the air etc.
4) Turbo core number:
It's also quite possible that the i5-4350u can one only ONE core at Turbo 2.9GHz while the others remain at 1.4GHz, whereas the i5-4258u might still run one core at 2.9GHz but the others are at its much higher base of 2.4GHz. Considering the higher TDP I consider this a likely scenario thus programs that are multi-threaded would run better.
So if all cores were used the relative difference might be:
2.9x1.3 + 2.4x1.3 / 2.9x1.3 + 1.4x1.3
(1.3 is guessing at thread performance of 30% of core frequency)
So the i5-4258u by this scenario might be up to 23% faster when all threads are used. I'm pretty sure that will turn out to be very close.
**It's also possible the i5-4258u can use BOTH cores at 2.9GHz which raises the performance up to 34% improvement over the i5-4350u.
So (roughly) the i5-4258u vs i5-4350u:
-23% to 34% faster CPU (all threads used)
-25% to 50% better GPU (educated guess until more info)