Taiwan has an unusually poor TOEFL performance. For its GDP, Taiwanese English fluency should be higher (Click here to read about GDP-English Fluency).
However, compared to other Asian nations, Taiwan does very badly (and compared to Europe, Taiwan is in real trouble.)
What is the explanation? The best comparison is between Taiwan, China, Korea, and Japan, since these countries have had similar exposures to English over the last 50 years. (On the other hand, Thailand, the Philippines, and Singapore have very different histories.) 
When we look at these countries' TOEFL scores from 2000-2009, we see an interesting trend. First, all scores took a major drop when the TOEFL speaking section was introduced. Test-taking cultures typically do poorly on the speaking section.
The real success story is Korea. In 2000, they were the worst of the group, with an average score of 66.5. Since then, they have passed Japan and Taiwan and will likely pass China in the next several years. They gained nearly 8 points over 9 years (compared to Taiwan's 2 point increase or Japan's 3 point drop).
Why was Korea so successful at increasing their TOEFL score? The answer seems to be part of a larger trend - a government/corporate effort to globalize their country. Over the last 20-30 years, Korea has focused on exporting their brands and culture to the world. And they know that English is a key to doing this.