Question

Why did Allies experience a slow start in the Pacific?

Answer 1

The Allies were unprepared for war and had a difficult time recovering from the initial Japanese successes.

Explanation:

America was ill prepared for war. The planes the US used in the beginning of the war were much inferior to the Japanese planes.
The United States Navy was smaller than the the Japanese navy.
Aircraft carriers would become the major vessels of World War II
The US started the war with only 3 Carriers while the Japanese Navy started with at least 7 carriers.

Then there was the major Japanese successes early in the war. The bombing of Pearl Harbor left the US Navy crippled. It took time for the US to rebuild the Navy.
The fall of Sinapore where 100,000 poorly led and equipped British soldiers surrendered to 30,000 war tested Japanese soldiers set back the British war effort. The Japanese had been fighting in China where their troops had became trained and experienced.

Also the major battleships of the British Navy were sunk by Japanese air power. The Japanese also had superior torpedoes. In one of the early sea battles, The Japanese used their torpedoes to sink four Allied Cruisers.

The superior industrial power of the United States would eventually wear down the early Japanese advantages. The battle of Midway resulting in the loss of four Japanese carriers to one American Carrier helped to even the odds in the Pacific.