Laser beam welding with coherent beam combining (CBC) enables spatial and temporal modulation of laser intensity. Material response to low-frequency modulation is well studied, but its response at high modulation frequencies up to the MHz range remains un-investigated. This study investigates the material response to shape frequency modulation spanning 0.1 kHz to 11.4 MHz under constant laser power and welding speed. Experiments were performed in full-penetration bead-on-plate configuration on 3 mm thick AA6005-T6 sheets using a dynamically shaped “Soother” beam, generated with a five-click CBC pattern. Mechanical performance, weld profile, porosity (X-ray CT), and high-speed weld pool imaging were evaluated. Four characteristic regimes were identified: dynamic capillary tracking (0.1–90 kHz), maximum instability window (≈90–180 kHz), hydrodynamic transition (≈180–900 kHz), and quasi-static thermal integration (≥900 kHz). Within the instability window, welds exhibited minimum tensile strength, maximum porosity, pronounced surface underfilling, and excess root penetration. Timescale analysis shows this instability arises when the total shape modulation cycle duration (5.5–11 µs) matches the capillary-inertial response time of the aluminium keyhole (≈6.5–11 µs for keyhole tip radii of 25–35 µm), promoting resonant amplification of capillary collapse and melt displacement. At frequencies exceeding 900 kHz, the cycle duration falls below the thermal diffusion length of the melt pool (LD ≈ 12 µm), forcing thermodynamic integration of the click sequence into a time-averaged spatial intensity distribution and restoring keyhole stability. These findings establish the ratio of modulation cycle duration to keyhole capillary-inertial response time as the governing parameter for weld quality in high-dynamic CBC laser welding.