= Solution
Without <surfactant>, <capillary pressure> drives a surface flow towards a growing bulge. <Advection> compresses the <insoluble surfactant> in converging parts of that motion, increasing concentration and lowering <surface tension>. The resulting gradient supplies a tangential <Marangoni stress> that opposes the surface motion. This additional resistance slows capillary-driven redistribution of liquid even after the base-tension decrease has already been accounted for.
In the strong-elasticity limit, concentration gradients suppress tangential interface motion. The surface behaves as effectively immobile at leading order. Setting $u_s\approx0$ in the <velocity> formula gives $\sigma_x\approx hp_x/2$, hence
$$
q\approx-\frac{h^3}{12\mu}p_x,
$$
one-quarter of the clean shear-free mobility $h^3/(3\mu)$. \b[This mobility reduction explains the quarter-rate limit; it slows the unstable capillary mode without removing the long-wave unstable band.]
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