AIRSPD DECAY AFTER MAKING XING RESTR. FMS CONFUSION.

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IN AN ATTEMPT TO MAKE A XING RESTR OF FL250; DURING CLBOUT; THE PF WENT TO MAKE CLB THRUST AND WENT TO VERT SPD CTL FROM PROFILE CLB CTL (EXCHANGING AIRSPD FOR ALT). AT FL260; AFTER MAKING XING RESTR OF FL250; THE PF RETURNED TO PROFILE CTL. WE WERE GIVEN A 15 DEG TURN TO THE L BY ATC AND THE PF RESUMED NORMAL CLB THRUST (I DID NOT OBSERVE THIS AT THE TIME). I NOTICED AND CALLED TO HIS ATTN THAT THE AIRSPD WAS APCHING THE MINIMUM SPD INDICATOR AND REACHED FOR THE VERT SPD CTL (WE WERE STILL IN A SLIGHT CLB). HE SAID THAT HE RECOGNIZED THE AIRSPD; BUT WAS CORRECTING. SINCE HE STATED HE WAS AWARE OF THE SIT AND THAT HE HAD IT UNDER CTL; I DID NOT OVERRIDE THE PROFILE WITH VERT SPD. THE AIRSPD CONTINUED TO DECREASE AND THE STICK SHAKER BECAME ACTIVE. THE NOSE WAS STILL ABOVE THE HORIZON AND THE STICK SHAKER AGAIN BECAME ACTIVE. THE ACFT NEVER STALLED; FULL FLT CTL WAS MAINTAINED; BUT THE REACTION TO THE DEGRADING AIRSPD WAS TOO SLOW. THE PF ASSUMED THAT THE PROFILE MODE OF THE AUTOPLT/FMS WOULD KEEP THE ACFT OUT OF ANY LOW SPD ENVELOPE. THIS IS A CASE OF TOO MUCH FAITH IN THE 'MAGIC' OF THE MODERN ELECTRONIC COCKPIT. THE HIGH ALT; LOW SPD; SMALL TURN; AND RETURN TO NORMAL CLB PWR SLOWED THE AUTOPLT/FMS RESPONSE AND CORRECTIONS. CREWS SHOULD NOT WAIT FOR 'THE SYS' TO INDUCE PROPER CORRECTIONS; BUT INSTEAD OVERRIDE WITH ADEQUATE AND TIMELY RESPONSES AS NEEDED; ESPECIALLY IN LOW AIR SPD; HIGH ALT ENVIRONMENTS. THEN RE-ENGAGE THE 'MAGIC.'