TheVaultDweller
Front line cannon fodder
Optimus (as well as most Transformers) are actually disproportionately fast relative to their size. They move with near human speed and agility despite being much larger.
YouTube video
Not to mention looking slow at giant speeds =/= being slow. They're actually covering way more distance in their motion than a human-sized opponent would in the same amount of time.
Or to give other examples, Giant-Man's Leviathan punch from EG. It almost looks like it's happening in slow motion from our POV, considering it takes him almost 2 full seconds to cock back his arm and throw the strike. Yet despite how it looks visually, his fist is traveling several dozen feet in those 2 seconds. How many people can cover that same distance in 2 seconds from a stationary position? No one without some kind of super-speed and/or flight powers. Or take Godzilla's tail whip. It doesn't visually look very fast, but his tail is also covering a couple hundred feet in its arc in a very short amount of time. Or Surtur in Ragnarok. It takes him like 4 full seconds to bring his sword down from over his head for the Asgard-killing strike, which is slow as fuk by human-sized standards. But at his size that sword likely dropped a couple hundred meters in those 4 seconds. So, not slow at all, actually.
It's how giant-sized characters like Kong, Godzilla etc. can tag helicopters, fighter planes etc. despite appearing sluggish from a human's perspective. Because they're not actually that slow. Their size just distorts our perspective.
That all being said, the Transformers have terribly inconsistent durability, which gets progressively worse and more inconsistent throughout the franchise, so not sure they'd handle the arsenal some of these guys have very well. Could possibly beat Ultron though, as Ultron didn't actually have the most impressive damage output.