Fatty is as slow as a sniper is compared to a titan. This is insane when the relationship between fatty and GC/chicken/I guess ML but not really is closer to percival vs sniper.
The speed of percy/fatty is a holdover from the days when UEF t3 stage was "ok so shift+g everything into a deathball and click enemy base/t4/acu"
Nowadays it doesn't work like that because the loss of 25% of the percy HP means aoe is now extremely powerful against them. The speed of t4s mean that you can see 25k mass in percies coming at you and just build an emergency mass dump t4 and, if those percies are split up in order to avoid the impact of aoe, will now not be able to consolidate in time and will instead engage the t4 5-9k mass at a time and instead of do damage actually gain it hp through vet feed.
Percies need to have a risk/reward mechanic revolving around speed so that you can punish defensive t4s through surrendering map control, consolidating at the right moment, and killing the t4. Consolidate too early and you risk aoe tools getting a shot at you. Consolidate too late and you vet feed. In current balance you either consolidate before you push and forever risk aoe death or you just don't push because T4s are 20% faster than percies.
Fatty is softly related to all of that. It faces the same punishments that sniper does (air, arty, tml) while having the speed of a percy against the equivalent of units that have the dps/tankiness of percies but the speed of titans. It's 1.75 speed against 2.5. This not only makes building up a critical mass of fattys (2-3) a pain in the ass because it takes 2 minutes to reinforce even on a 10x10 map, but it makes it insanely easy to scout it super late and still build up a decent defense against it. If a GC was 1.75 speed it would also be extremely easy to kill because you build up a dump of 20 pd and 2 shields and it autodies. It tries to turn and attack around the PD and by the time it reaches that point, it's been 2 minutes and the same defense is up again.
The balance of percy/fatty is fine on a unit relation level, the problem is that it is not synchronized with the change in UEF doctrine to be about constant gorilla push and pressure at the t3 stage. UEF doesn't have the time to sit down and do nothing while it builds a death ball.
Just as a spitball I'd do smth like
Percy 2.1 -> 2.2
Brick stays 2.3
Chicken/GC 2.5 -> 2.1
ML 2.5 -> 2.2
Fatty 1.75 -> 2.1 (you can argue higher or lower here honestly, but it should be around the same as other direct fires)
Mega is 2.0 which is fine tbh, though I don't mind it also going 2.1 if people want.
Then you can buff harb and othuum to 3 and 2.9 respectively to eat a bit into the titan/loya insane speed as compensation. Or you can keep it the same since that brings harb back to the speed in pre t3 rebalance. No idea what is good.