Very slow to create one figure

[mfouesneau]

I have the issue that even the examples are very slow to run (multiple minutes)

#include "xvega/xvega.hpp"
using namespace xv;
auto df = url_data().url("https://vega.github.io/vega-datasets/data/cars.json");

auto x_enc = X().field("Miles_per_Gallon").type("quantitative");
auto y_enc = Y().field("Horsepower").type("quantitative");
auto enc = Encodings().x(x_enc).y(y_enc);

auto mp = mark_point();
auto fig = Chart().data(df).mark(mp).encoding(enc).title("Simple 2D Chart").width(400).height(300);
fig