[tknarr]

I've maintained ProfitUI, and it doesn't use /do_file at all. I've also raided, and never see any external software (ACT, RaidHub and so on) cause lag. The only time I've seen /do_file used in fact is in ACT to report parses, and that's usually done after the fight is over. It's possible to build files with commands that'd cause lag and execute them via /do_file, but it's more common for those same commands to be executed manually since they're just the correct reaction to an event during the raid.

The biggest causes of lag that I know of are:
1. Effects during raid that cause cascades of proc checks, eg. an ability used by the mob that when it lands on a player puts a detrimental on them that procs, causing more checks and when it lands causes more procs which cause more checks etc. etc..
2. Particle effects from group-effect spells, since raids have more players and pets active than normal groups and so cause group-effect spells to generate more effects. Stuff like this is why raiders play with minimal graphics quality and aim the camera at the floor, and why Profit and other custom UIs have buttons to switch between graphics configurations easily.
3. Effect mechanics that've been abandoned because they caused too much server-side calculation, eg. the de-leveling effect in Mistmoore raids.