Just a slight difference. Lengthy seems to be used a lot when talking about time frames, especially a time frame which is longer and maybe more tedious than usual.As a first point, if I replace lengthy with long in the context, would it cause a change in subtle nuances?
It's not necessary, but would give a change in nuance.Second, is it necessary to insert levels of after highest?
"..needed lengthy periods of instruction and practice to achieve their highest performance" seems to refer only to the persons highest peak performance.
"..needed lengthy periods of instruction and practice to achieve their highest levels of performance" suggests various high levels that are not necessarily at the very top or peak.