An unpaired Electron does not set a universal reaction rate. A stable radical can retain radical character during isolation and storage under specified conditions; a persistent radical survives long enough under specified conditions to be observed. These descriptions concern lifetime and handling, rather than immunity to every possible chemical reaction. DPPH is a practical counterexample to the claim that all free radicals react immediately.
Two mechanisms help explain persistence. Spreading electronic character over several chemical atoms, represented by chemical resonance, can lower the energy relative to a localized description. Bulky surrounding groups can impede approach to a reactive site and slow recombination. Lower energy and slower chemical kinetics are different advantages: neither alone guarantees a long lifetime. A primary computational study explicitly separates these effects, using electronic and geometric descriptors; it does not directly measure every predicted lifetime.[ref]
A quantitative metric for organic radical stability and persistence using thermodynamic and kinetic features by Shree Sowndarya S. V. and others. A quantitative metric for organic radical stability and persistence using thermodynamic and kinetic features, Shree Sowndarya S. V., Peter C. St. John and Robert S. Paton, 2021: Abstract and Introduction, thermodynamic stabilization versus kinetic persistence
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