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.
Sun did have a firewall by the early 90's. It had application-level proxies, and you'd have to configure applications to bounce through it if you wanted to get to the Internet. In many ways, this was more secure than today's default for firewalls where you can make any outbound connection you want but only the inbound connections are filtered.
Note that I'm not arguing that Sun was a leader in security, but they did make some efforts that other companies didn't.
https://www.govexec.com/federal-news/1999/02/postal-service-...