I actually did watch the video (which would be of higher resolution and easier to discern if shot in landscape
). We will have to narrow down the possible causes for your issue. Your computer should be more than capable of handling audio without any glitches.
1. Are you running the latest Tascam firmware form February 2020?
2. Can an ASIO driver be selected for the 16x08 in Reaper's audio device preferences? If yes, select it.
3. Can you access the Tascam's own system panel / preferences via the "ASIO configuration" button in Reaper's audio device options page? If yes, are these setting matching those in Reaper (also in Reaper project properties)? What's the current buffer size?
4. Did you try with "use audio driver reported latency" checked and another time with this option unchecked?
5. Did you try to run the tests with a fresh, portable Reaper install?
.