What was Tony hurrying to finish...

Hard to say with certainty, but I always assumed Tony was trying to disable some sort of alarm which would've been triggered by the phone lines getting cut by Karl.
The two brothers do exchange some German dialog before Karl starts cutting - perhaps someone with some knowledge of German can translate for us?

[deleted]

It was my impression the phone cables would have electrocuted Karl had he cut through them with the chainsaw, given they connect so many phones throughout a huge office building, so his forcing Tony to hurry was due to Tony caring about his brother, and is why Tony is so relieved and then looks angrily at Karl afterwards, while Karl clearly just likes to *beep* with people. The scene would therefore be a reveal that they are bros who care about each other despite being murderous *beep* which adds context later when McClane happens.



[truth]I am bisexual. If you dont like that: you can blow me. If you do: same deal.[/truth]

I believe Tony was trying to disable the alarms HIS way, to demonstrate his expertise. Even though it was a time-sensitive issue, he didn't want Karl to take away his moment in the spotlight by shortcutting the job.

no idea.. but that could've seriously impacted the mission?!

such a stupid scene to establish weird rapport between the brothers..

Tony was trying to bypass the alarms system. Karl cut through the cable to disable communication. As another post pointed out it would probably have been triggered the alarms if the cables were cut before the bypasses were complete.