In order to do it quickly, you will need to delete the SSISDB and recreate it. But I don't recommend doing that. The cleanup job is interfering with your execution of SSIS Packages (or vice-versa).
My approach when I'm trimming back the retention period is to walk backwards a day at a time. The script below will do that, just change the @DesiredRetention to what you want and it will walk backwards to that point. You can stop and start this script multiple times (to allow for it to run when packages are not) and it will pick up where it left off.
DECLARE @DesiredRetention INT = 30;
DECLARE @CurrentRetention INT = (SELECT property_value FROM SSISD.internal.catalog_properties WHERE property_name = 'RETENTION_WINDOW');
WHILE @CurrentRetention > @DesiredRetention
PRINT 'Removing History older than: ' + CAST(@CurrentRetention AS VARCHAR(10));
--Change the desired retention period...
EXEC [SSISDB].[catalog].[configure_catalog] @property_name=N'RETENTION_WINDOW', @property_value=@CurrentRetention;
--Run the Cleanup Job
--Decrement the value.
SET @CurrentRetention = @CurrentRetention - 1;
--Make sure to set the desired retention period...
EXEC [SSISDB].[catalog].[configure_catalog] @property_name=N'RETENTION_WINDOW', @property_value=@DesiredRetention;