Compared to a HDD, yeah USB 3.0 speeds aren’t too bad, but most laptops being released these days use an NVMe for storage (or possibly even a soldered drive). My comparison was around what you’d expect in a laptop purchased in the past year or so.
For your partner’s laptop, getting better read performance from an external drive doesn’t surprise me, but there are also limits to this. Games are starting to support DirectStorage, which allows the GPU to directly read and decompress assets from the hard drive. This won’t work with an external drive (at least from my understanding), so those games will likely fallback to much slower methods of loading assets if they support the laptop at all. This is also not taking into account the other hardware on the laptop, which might have been excellent for the time, but with how much CPUs and GPUs have advanced over the past 4 years, I wouldn’t be surprised if they’re starting to reach their limits with today’s major releases.