Using time values from this calc: https://vsbattles.fandom.com/wiki/User_blog:The_real_cal_howard/Mario_crosses_World_5

Screenshot from the Super Mario Wiki

Worlds in Mario Galaxy 2 are called universes, so Starship Mario is likely crossing a universal-distance instead of just a galaxy cluster, so 93.016 billion lightyears

Low-end: 1 hour

93.016 billion lightyears = 8.80024376 * 10^26 meters

(8.80024376 * 10^26 meters) / 3600 seconds

2.4445121556 * 10^23 m/s or 5.4682270212 * 10^23 mph

815,401,485,370,000c (815.4 trillion c)

High-end: 10 seconds

(8.80024376 * 10^26 meters) / 10 seconds

8.80024376 * 10^25 m/s or 1.9685617276 * 10^26 mph

293,544,534,730,000,000c (293.54 quadrillion c)

This is also a lowball since parallel dimensions are stated to be infinite in size several times, which would make the main Mario universe infinite as well (hence the “parallel”) so this speed would likely be infinite.