I wouldn't expect much.
Even Apple said the CPU increase was 25% while GPU was 50%.
But upgrading from an iPhone 4 with the A4 chip...the iPhone 6 is going to be much for me.
Technically they said "up to" 25%/50% faster. Which could mean only for a very few small applications (we don't know). The average increase is likely to be much less. In other words, it's probably "a little" faster, which personally I am OK with because the 5S has good performance and if I can get that performance with a much larger screen, I'll be happy. I'm upgrading from a 4S so it should be a massive boost for me anyway.