on HDX there is an error that occurs occasionally where a very noticeable delay randomly appears on record enabled tracks when recording or even when transport is not running. It seems to be ADC related and was not present on older HD systems.
As this issue appears both on ProTools HD 10 and 11, it seems to be hardware (firmware) related.
There have been various reports of what may be the cause, including certain (non-DSP) AAX Plugins, duplicating (record-enabled) tracks, reaching a certain number of tracks in the session... although no one has been able to reproduce the error consistently.
the current workarounds are disabling and re-enabling ADC (does not work for some users), going in and out of input monitoring, deactivating and re-activating the specific track, closing and re-opening the session or restarting ProTools.