There are a couple of issues to deal with at once. NIDAQ function calls are not made until any current function calls have returned. If you have parallel calls to the NI-DAQ driver, one of them will be waiting for the other to complete.
Also, a loop will wait while a call in a parallel loop has not returned yet. This is not improper operation, just a consequence of LabVIEWs 'dataflow' style of execution.
How to fix? Bigger DAQ buffers, code streamlining, and structuring your program to avoid time-dependent/critical sections of code from having to wait for other processes.