There's two things here.
The block size is the system lag you set.
When you need to hear the output from the computer in time with the live here and now real world, the output from the computer must be no greater than 11ms latent. That's the threshold for perceiving a delay. Like drifting out of sync with the lips vs the voice on TV.
When you set the system lag to do that, the computer needs to crunch all the processing in that lag time you set. The system of the audio interface and computer either handles that or not. Low powered computers or laggy USB audio interfaces are thus not able to be used for live sound scenarios.
That's the deal. It takes time for the audio to clock through the computer.
Because making the computer scream for live audio might be hard (and used to be impossible)... audio interfaces include built-in mixing boards to let you monitor live inputs with no lag - bypassing the computer. That lets you set the block size large again to allow for more processing because we aren't doing the live sound part with the computer.
All good. Simple workaround in a way.
But now the incoming audio is REALLY offset.
(It's actually slightly offset in the nearly-zero low latency mode too. It's whatever the lag set with the block size is.)
The calibration I mentioned earlier is to make Reaper align the incoming audio with the existing audio to normalize for the system lag.
You weren't complaining about hearing a lag when monitoring yourself...
So I'll guess you were using the mixer built into your interface to monitor live inputs. Then you set the block size lower. Two wrongs made a right in that the recording offset was lessened by the lower block size. If you're monitoring your live inputs with your interface though, you'll get more CPU power left for mixing leaving the block size set high (512 or 1024 samples) and fixing the calibration issue in Preferences/Audio/Recording.
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