Some of the examples are really much simpler than they appear: they have multiple consonant-letters but less actual sounds. For example, in "phthisis", there are only two "consonant" sounds together. The "ph" could be replaced by "f",and the "th" by the old letter thorn, giving "fþisis", with only two letters together.

I agree that "Thou triumphedst" is unlikely to have been pronounced as its spelling would imply. The same usually goes for other combinations such as "next step". As the "x" already represents two sounds, it could come out as "nekststep" but in fact is usually "nekstep".

Richard