By Alina Selyukh WASHINGTON (Reuters) – The chief executive officer of Japans SoftBank Corp on Tuesday called the U.S. wireless market an oligopoly plagued by slow speeds and high prices and said his companys Sprint Corp could shake up the competition, but it would require a scale that Sprint cannot reach alone. Masayoshi Son, the billionaire CEO of SoftBank, in his first public speech to a U.S. audience since his company gained control of Sprint last year, lambasted the U.S. wireless market as offering pseudo-competition. Son did not directly speak about his efforts to engineer a merger of Sprint and T-Mobile US Inc, the No. 3 and No. 4 U.S. wireless carriers, but told reporters he hoped to meet again sometime with U.S. regulators, who so far have given a cold shoulder to such a deal.