Sorry I've taken so long to reply - life has a habit of getting in the way.
I have a triple boot system ,with Windows 10, Solus and (formerly) KaOS. I hadn't booted into Solus for a while, but noticed the same behaviour there. With Nvidia or Nouveau, same delayed boot time.
I rebooted to go into Windows 10, which I hadn't had problems with previously and got a SMART error which I hadn't had before - I remembered I'd just installed a new SSD for the Windows 10 side so took my case side of and checked connections - I had managed to knock one of the sata connectors loose. Reconnected all those securely, and restarted my machine.
Still had the delay on boot in both Solus and KaOS, but it wasn't consistent. I could boot from cold into either and have no delay. If I restarted and went into the other - delay was back. I then began to wonder is the new SSD had an issue, as I had installed KaOS around the same time as the new SSD.
Found that there was a firmware update for the SSD I had bought, installed it and rebooted - I no longer have any delays in any operating system. I had installed Antergos to test which had the same issue, but all now running smoothly.
Just thought I'd let you know. Cheers for the help.