Technically, if ships build at a faster rate, shouldn't you have a slightly larger fleet as well?
your fleet is still gated by income and supply limits. And if income is also set to fast, the titan will be made at a proportionately faster rate.
But in all honesty hitting supply cap before the enemy starts work on the titan is easy(after all the opponent has had time to gather 13k+ credits+crystal/metal, research 4 research topics, and build a titan foundry). The big difference is that because multiple frigate factories can be pumping out frigates every 20-30 seconds, the major restriction on frigate production usually isn't production speed but rather resource and supply limitations(titan production speed is more of a gating feature because of the long production time coupled with the fact that it can't be sped up by building multiple foundries).
The only time the improved production speed may help counter a titan rush is if you for some reason have to produce a fleet to attack the foundry(rather then already having an active fleet), which is essentially you getting caught with your pants down anyway(if by the time an opponent has enough for a titan you don't have a good standing fleet you should be vulnerable to a rush).
I'll concede you may have an couple ships for the assault sometime if you were slow producing a fleet or have a high fleet supply, but honestly you'd need enough extra ships to increase your fleet's total firepower by about 26% to make up for the shorter window of opportunity and have the same chance of destroying the foundry before the titan rolls out. But again, in the time it takes the opponent to gather all those resources, you had ample time to have maxed out your fleet supply long before the opponent started building the titan. With unlimited resources and supply you would be right- but for simply building a large fleet supply/resource costs are much more a an inhibiting factor then production time. By contrast the production time of a titan is very significant As it is meant to be a time of great vulnerability against opponent who have simply built a large fleet.
In point of fact this is probably part of a larger pattern: improve production speeds in general are a greater advantage to the defending player then the attacker(after all their new units start closer to the front and have defenses to hide behind until there are enough of them to group up). Countering titan Rushes is however one of the most extreme examples due to the nature of titans.
