There are two factors:
The propagation speed (or velocity factor VF as a fraction of the speed of light c0) depends heavily on the permeability of the cable's dielectricum. Essentially, the copper only guides the electromagnetic field that propagates through space. The VF is calculated by VF = 1/sqr(k), with k being the permeability of the cable (conductor and insulator/dielectricum combined). Coax or twinax cable can have very 'fast' insulators, with up to and even more than .9 VF.
Cat-5/6 twisted pair has a VF of only .65 (Cat-7 is around .75), so the electrical signal simply takes longer to cross the cable. Given the short lengths of especially twinax cable this is pretty negligible though - for 15 m, the maximum twinax reach, the difference is ~20 ns.
A much more significant effect comes from the line code. While twinax works with simple 64b/66b (for most port types) which has next to no encoding overhead, 10GBASE-T requires elaborate encoding[1] which causes an port-pair overhead of .8 to 2.5 μs (depending on the generation and grade of the port hardware).
That encoding overhead is also the reason why fiber is considered faster than twisted pair even though the signal itself might actually propagate a tiny bit slower.
Speaking of 100 Gbit/s - there's only the short-reach 40GBASE-T for twisted pair and that's the end of the line there (permanently). Everything faster uses fiber or (very short) twinax/DAC/-CR.
[1] 10GBASE-T uses Reed-Solomon forward error correction and Tomlinson-Harashima precoding, producing considerable overhead (for Ethernet), in addition to PAM-16 modulation with DSQ128 "checkerboard-pattern" symbol selection.