What would you recommend then? I was hoping to get an external SSD HDD that would be speedy.
The issue is that for a peripheral bus (SATA, USB, PCI, etc.) controller to be effective, it has to be attached directly to the SoC. On a board like the Pi (or pretty much anything else, including normative laptops, desktops, etc.) you cannot add to what is already there in this sense. You cannot wire anything directly to the processor.1 You have to work with what is there.
Hence, people make SATA adapters for the Pi, but they are really just SATA -> USB adapters (except they are noticeably more expensive -- go have a look at a generic SATA to USB adapter online). The USB hub on previous models of Pi was actually the ethernet controller too, meaning 100 Mbit ethernet had to share a bus with 280 Mbit USB where the limitation was the latter and it was shared. The Pi 3B+ is an improvement on this in that it has an independent Gbit ethernet port.
This means the fastest port on the 3B+ is no longer USB, it's the ethernet port. So if you wanted maximum throughput, it could be by connecting directly via ethernet cable to another system which can properly house SATA drives, and you might, at least in theory, get speeds close to 100 MB/s, although whatever network protocol you use will take a chunk of that. All things considered, this is probably not a worthwhile solution.
Short of that you just have to be happy with the USB. Since any new harddrive you buy today will be capable of matching that bottleneck, there's not much point in buying one that's faster than anything else -- attached to a USB 2.0 port, they're all likely equal. The one thing I would recommend is that if you buy one that doesn't have it's own power supply, don't get anything too beefy, and definitely do not get more than one. I.e., if you want multiple drives, get ones that have independent power supplies (or you could use a powered hub, but this might add to the bottleneck).
1. Actually that not true -- the stuff on the GPIO breakout is connected directly to the SoC, and in that sense you can attach things directly to it. However, nothing on the breakout has the potential to rival USB 2.0 speeds.