Slow to start with FileVault enabled
Since I upgraded my iMac Late 2013 to Sierra, it takes a long time to recognize my Magic Mouse after startup, and the wireless keyboard is even slower to get going. It doesn’t run any old software, but I did convert it to FileVault just before upgrading. Is this just Sierra? Both Sierra and High Sierra alter the timing used to connect to Bluetooth devices during the startup process. This may result from EFI firmware updates, which are applied during the upgrade, and the delay is also set in NVRAM, although you shouldn’t attempt to change it there.
Check that your Mac is running an appropriately recent version of its EFI firmware. You can find this out by clicking About This Mac in the Apple menu, and then clicking the System Report button. In the Hardware Overview, the firmware version is given as the Boot ROM Version, of which the last two groups of hex digits give the version. Yours should read at least 0118 B42 in Sierra, or 0123 B00 after an upgrade to High Sierra. A full list is available at bit.ly/efi_firmware.
FileVault’s disk encryption won’t help your iMac’s speed, particularly if the encrypted startup disk isn’t an SSD. It’s worth thinking carefully about whether you need the whole disk to be securely encrypted, or whether it would be better just to protect its most sensitive files. When High Sierra is mature enough, you might find that its encrypted volume support is both quicker and more appropriate for you. If you don’t need whole disk encryption, now is a good time to get rid of it.