Without salts: Generate table of hashes vs. Plaintext for all plaintext up to certain length. Maybe use rainbow tables. Now, for each entry use this table. Time to get plaintext of n hashes is constant.

With salt: Generate table of hashes vs. Plaintext by appending salt to each possible plaintext of given length. For first entry use first salt to generate this table. Table is now useless for second salt if it is different. Time to attack n hashes scales linearly.