I have not used their services, however I’m old enough to have used standard and high speed duplication the first time cassettes were popular.
High speed definitely sounds worse. It’s totally fine for speeches (many church service sermons, for example, were what kept tape duplicators in business) but if you want a great sounding cassette you won’t get it from high speed. But also, it won’t sound dreadful, just not as good as standard speed. This is due to physics, not any particular company’s practices.