At startup my computer pauses annoyingly for about 10 seconds, during what I think is called the POST screen. I mean the screen where it says "Press ... to open BIOS settings".

This has been happening for a long time - but before that there would be the usual ~1s pause.

My BIOS settings are default except for RAM frequency and ignore-CPU-fan-speed.

I have Arch Linux and W10 both installed on separate SSD's (both Samsung 860 EVO). ASUS Z87-A motherboard.

How can I get a normal pause back?

Is this at grub? Have you changed the timeout?
https://wiki.archlinux.org/index.php/GRUB

If it's the setup then I would think that some setting in System Boot Timeout needs to be changed.

I don't think it's GRUB, it's the motherboard loading screen. I could be wrong but I think GRUB is the screen that lets me select whether I want Arch or Arch fallback - plus a command line.

Do you have any delay for IDE/HDD detect enabled (in system BIOS)? Is the system doing any memory testing (per system BIOS)? Have you recently added memory?

If you push 'Tab' during this screen does it show you any text? If so, what does that text say?

Unless there is a drive that's failing (which can be checked with SMART... enabled in BIOS) the most common cause of such a long delay is a setting that tells the BIOS to check RAM thoroughly, which means slowly and more than once usually. Best bet is to enter your BIOS Setup and page and scroll through to look for a startup setting like that. Do check your drives though.

Incidentally most owners manuals go over every single setting in BIOS to explain what they do. If you don't have a manual it is highly likely you can download one from the manufacturer's Support page.