Regarding players coming from the US, there was also Elisabeth Moore, but she didn't enter Wimbledon. She played other tournaments in Ireland (I saw at least one in the results thread). It was before 1900. And like Marion Jones at Wimbledon, didn't go far, despite being a US champion (Moore will win the USC four times, in 1896, 1901, 1903 and 1905). Which is certainly an indication of the inferior level of US players compared to UK ones by then. That indication also worked the other way around: Irish player Mabel Cahill was twice US champion in 1891 and 1892, whereas she barely won any event in England before (she won a few small ones). May Sutton was the first US player to play up to the best players of England. It appeared she even was the best in the world (first winning everything on the West coast, then on the East coast, then in England!), along with Dorothea Douglass. I recall reading Sutton said the English player who impressed her the most was Charlotte Cooper.
May is probably comes out the worst when it comes to slam numbers showing a players worth.
She was so much better than just 3 slams.
Her extreme topspin, schooled on California hard courts, rolled over the opposition.
It took a combination of high skill, someone who could break up her game, and a wet court to beat her. On a slick court her spin didn't blast throw, and a good net rusher could take advantage.
Two players who beat her this way were Haze Wightman and Chattie Stery, fitting what Hughes posted about
Sutton said the English player who impressed her the most was Charlotte Cooper