The present invention relates to a method of increasing the high-speed performance of a carcass structure in a tyre for vehicle wheels. The method includes: preparing at least one hybrid yarn (13) and assembling a carcass structure (2) comprising at least one carcass ply (3a, 3b) substantially built on cords (12) comprising the hybrid yarn (13). The hybrid yarn (13) comprises a plurality of filaments (14a, 15a) obtained from a first multi-filament yarn (14) of polyester and a second multi-filament yarn (15) having a decay of the modulus with the temperature lower than 20%. Each of the first multi-filament yarn (14) and second multi-filament yarn (15) comprises a plurality of individual filaments (14a, 15a). The individual filaments (14a) of the first multi-filament yarn (14) are at least partly mixed in proportions of at least 50% with the individual filaments (15a) of said second multi-filament yarn (15).