Well then you are using the wrong physics.

The expression for kinetic energy of a free massive particle in special relativity is $$E - E_0 = mc^2 \left( \left(1 - \frac{v^2}{c^2} \right)^{-\frac{1}{2}} - 1 \right) = mc^2 \frac{c - \sqrt{c^2 - v^2}}{\sqrt{c^2 - v^2}}$$

Let $$x = \sqrt{c^2 - v^2}$$ then $$\lim_{v\to c} E(v) - E_0 = mc^2 \lim_{x\to 0} \frac{c - x}{x} = \infty$$

So there is no velocity-limited upper limit to kinetic energy in special relativity.

But since atoms break apart at energies of a few eV, in the limit of high energies, everything is (normal) plasma and then quark-gluon plasma and these electrically charged (and then chromodynamically charged) particles give rise to strong fields which give rise to pair-production of particles. So at high temperatures, thermal equilibrium means normal matter and antimatter should be at near equal levels and the rate of pair production should equal the rate of annihilation.

The problem is the universe is not 50% antimatter so dense normal matter is a barrier to getting very, very hot.
https://en.wikipedia.org/wiki/R136a1
https://en.wikipedia.org/wiki/Pair-instability_supernova