Does online backup using BAR which behind the scene running multiple streams of arcmain effect LOAD jobs as it applies HUT locks some times. Will it slow down the LOAD jobs ?
Any two operations on the same system that will compete for resources will always have the potential for impact, even assuming you aren't backuping up objects that are also being targeted by loads at the same time (which you shouldn't).
Teradata Online backups don't use HUT locks however they require read locks to create the subtables
ETL jobs that create and distroy objects rather than reusing load database/tables can create conflicts within DBC which Online backups need access to DBC tables.
Online backups generate more I/O than standard backups because the before image of altered rows need to be copied to the subtable which is backed up once the base object is backed up.
Running bakcujps while data is being updated is what online backups are for. The salient point is to add enough space for the journaling. When an online backup fails for lack of space, it continues to journal until you manually turn the online archiving off (LOGGING ONLINE ARCHIVE OFF FOR <database_name>;). The journal is in the same database as the data being backed up, so once it reaches maxperm, any updates will also also fail.