What are the ground and flight requirements for high performance endorsement?
You seem to be confusing a high-performance endorsement with a high-altitude one. Both are in 14 CFR 61.31. They're too long to quote here in full, but the high-performance one comes down to this:


  (i) Received and logged ground and flight training from an authorized
  instructor in a high-performance airplane, or in a flight simulator or
  flight training device that is representative of a high-performance
  airplane, and has been found proficient in the operation and systems
  of the airplane; and


The high-altitude one is where your 25,000' number comes from:


  (g) Additional training required for operating pressurized aircraft
  capable of operating at high altitudes. (1) Except as provided in
  paragraph (g)(3) of this section, no person may act as pilot in
  command of a pressurized aircraft (an aircraft that has a service
  ceiling or maximum operating altitude, whichever is lower, above
  25,000 feet MSL), unless that person has received and logged ground
  training from an authorized instructor and obtained an endorsement in
  the person's logbook or training record from an authorized instructor
  who certifies the person has satisfactorily accomplished the ground
  training. The ground training must include at least the following
  subjects: [...]


As well as the ground training, you have to do (at least) the following in an aircraft or approved simulator:


  (i) Normal cruise flight operations while operating above 25,000 feet
  MSL;
  
  (ii) Proper emergency procedures for simulated rapid decompression
  without actually depressurizing the aircraft; and
  
  (iii) Emergency descent procedures.