The Delta brake seems to be one of those things that one either loves or hates. Contributing to the latter is the somewhat ineffective stock brake pad. I learned years ago that with a little bit of coaxing, a Nuovo Record-style pad will fit into the holder. I'm running mine with Kool Stop "Campi" salmon pads, and they work much better than stock.
back in the day the trick was to sipe the pads they grabbed a lot better as it softened them up to grab and actually slow down the bike as intended for consumers , a lot of pros preferred just the rear be siped slightly ( one or two cuts)