The magnitude of the variables does not matter. For a computer, the addition of pi + 1.23456789, pi + 1.0 and pi+ 1.23456789e37 takes exactly the same time. Therefore the simplification does not accelerate the evaluation. And by the way, this is very fast in Matlab at all:
fx = -(1.0*(1.82e+39*x^2 + 1.81e+32*x - 3.04e+39))/(6.35e+41*x + 4.27e+37);
fx = -(1.0*(1.82e+7*x^2 + 1.81*x - 3.04e+7))/(6.35e+9*x + 4.27e+5);
Both take about 0.4 seconds on my i5-mobile Matlab R2018b. A tiny acceleration is measurable, if you omit the meaningless multiplication by 1.0.
The slow computation has another cause: ODE45 is a solver for non-stiff equations. Use a stiff solver for this equation instead:
[t,x] = ode23s(@(t,x) -(1.0*(6.84e+45*x^2 + 5.24e+32*x - 2.49e+42))/(2.47e+39*x + 7.12e+37), tspan, x0);
There is a very sharp knee at the start. ODE45 struggles massively with it, because the stepsize controller drives crazy.