It's important to point out that these programs have two different objectives.
The Mullvad client is designed to connect to a closed-source service, which is run by someone else. It supports a bunch of different plugins, including openvpn and WireGuard. So probably it could adopt rosenpass, at least with its WG plugin.
WireGuard is designed for minimal protocol variability, high assurance implementations, and ultra small code size. It's used by VPN services, but also by end-users creating their own tunnels.
The slowness was due to a hardware bug in the 6522 VIA chip. The shift register (FIFO) would lock up randomly. Since this couldn't be fixed before the floppy drive needed to be shipped, they had the 6502 CPU bit-bang the IEC protocol, which was slower. The hardware design for the 154x floppy drive was fine, and some clever software tricks allow stock hardware to stream data back to the C64 and decode the GCR at the full media rate.
https://www.govexec.com/federal-news/1999/02/postal-service-...