Two piano performances heard this past weekend: * On Friday, Alexander Toradze attacked Prokofiev's Piano Concerto No. 2 with the Los Angeles Symphony with enough zeal to get half the audience on their feet. Mr. Toradze ''merely'' won the silver medal in the 1977 Van Cliburn competition. So only one- third of Symphony Hall was filled - absent also the draw of director Carlo Maria Giulini, who was unable to perform. Toradze seized - somewhat questionably - the opportunity to stretch his time in the limelight by playing an encore - an extreme rarity for concerto soloists.
* Sunday at the French Library, local pianist Virginia Eskin virtually assaulted her repertoire. Playing in the library's splendid Candelight Series (parlor replete with gilded wood paneling and electric candles), she did not allow her 80-or-so listeners to relish her romantic waltzes and polonaises (Brahms, Chopin, Liszt). She raced through at breakneck speed as if she had a dinner date to meet. Undaunted by garbles and sloppy phrasing, she seemed determined to keep the tempos faster than her ability to articulate them. The result seemed aimed - however failingly - at impressing her captive audience.