That is expected. The default patch is designed for operation at 1.67 GHz. So if you operate it at 5.8 GHz, the wavelength is much smaller, which results in more number of elements in the mesh -(surface and volume discretization of the structure). In a sense you could say the structure itself looks bigger at that frequency by approximately a factor of 5.8/1.67. Bigger mesh, means that the solver takes more time to run to solve it.
You could start off by designing your antenna at 5.8 GHz with a dielectric using "design" function in Antenna toolbox in the following way:
clear;
tic;
%%basic parameters
fc = 5.8e9; % central frequency (Hz)
%%patch element
substrate = dielectric('FR4');
patchElement = patchMicrostrip('Substrate', substrate);
patchElement = design(patchElement,5.8e9);
figure;
pattern(patchElement,fc);
toc;
