AN ACR FLT DSNDS THROUGH ITS ASSIGNED ALT AS IT LOSES AIRSPD DURING APCH TO SAN; CA.

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THIS FLT WAS 5TH LEG OVER AN 11 HR DUTY DAY WITH 6 PLUS HRS SCHEDULED FLT. NEVER FLOWN WITH THIS FO UNTIL PREVIOUS DAY. WATCHED HER SO MARGINAL JOB EARLIER ON ILS RWY 28R SFO. NOW HER TURN TO DO LOC RWY 27 SAN IN IMC CONDITIONS. HAVE SINCE DISCOVERED FO CONSIDERED WEAK BY OTHER CAPTS. FO SENIOR ENOUGH TO BID CAPT AND HAS NOT DONE SO. FO FLYING ON AUTOPLT AND CTLING HDG/SPD/ALT WITH MCP. THIS WAS SAME SCENARIO EARLIER WITH THE BOTCHED SFO ILS; SO I WAS WATCHING MORE CLOSELY. INSTEAD OF CONTINUING OUR VECTOR TO INTERCEPT LOC 27 OUTSIDE SWATT INTXN; APCH CTLR ISSUED ONE MORE TURN TO INTERCEPT INSIDE SWATT; BUT NOT LOWER THAN 3800 FT UNTIL ESTABLISHED. I OBSERVED FO SELECT CORRECT HDG/ALT ON MCP. THEN I LOOKED DOWN TO CHK APCH PLATE FOR NORMAL SWATT XING ALT. AS I RETURNED TO CHK FLT INSTS; I SAW AIRSPD AT 160 KIAS AND SLOWING. WE WERE AT FLAPS 1 DEG AND SHOULD HAVE BEEN AT 200 KIAS. I CALLED 'AIRSPD!' AND SELECTED FLAPS 10 DEGS. FO DISCONNECTED AUTOPLT; SHOVED UP PWR AND CONTINUED DSCNT TO 3500 FT. NOTICED SPD DIVERSION AT 4000 FT. WE QUICKLY RECOVERED TO 3800 FT. FO THOUGHT AUTOTHROTTLES WERE CONNECTED; SO AS SHE BEGAN TO SLOW DSCNT APCHING 3800 FT; AIRSPD DECAYED. FO'S LOSS OF SITUATIONAL/AUTOMATION AWARENESS AND MY HEAD DOWN PLATE REVIEW RIGHT AS WE WERE TO BE LEVELING CONTRIBUTED TO ERROR. FATIGUE WAS ALSO A FACTOR.