The one installed by "xfe-2.1.5-install-linux-amd64.run". One computer is AMD Ryzen 5 5500GT with 2TB NVME. Second computer is an old Intel Core i5-3570K with 500GB 2.5" SATA SSD. Copying has been done over various storage types - USB, SAMBA shares, local disks. Behaviour seems the same and not affected by the source/destination storage type or bus.
Same issue affecting my systems, both Kubuntu 24.04 LTS. Speed reduction copying files is dramatic - at least 90%. Had to (reluctantly) use another file manager (Krusader usually) when copying/moving files. Reinstalling 2.1.3 fixed it.