All good soaring pilots cultivate a near obsession with climbing as fast as possible whenever they can. In large, powerful thermals this may mean forsaking gentle circling technique in smooth, easy lift for the challenge of working turbulent cores for measurably faster climbs. Finding stronger lift and staying in it longer: such small triumphs facilitate further improvement in every area of soaring.
With increasing experience you’ll learn to sense from moment to moment which speed and bank angle are most appropriate for any given occasion. Then, where a beginner may have challenge enough simply maintaining full control in uniform circles the seasoned pilot might intentionally vary speeds and attitudes, perhaps more than once every circle in reaction to each thermal’s ever-changing behavior.
If a thermal is small or weak such an effort could be necessary to avoid falling out of it entirely. For according to our model of a bubble rising like a vertical smoke ring, lift is typically weaker near a thermal’s edge. Although the bubble itself may continue to rise, if your aircraft sink rate is greater than the vertical speed of that outer portion you might eventually descend through the bottom of the bubble while it continues on up without you. Aggressively working to stay in the core, where the lift is strongest, may be able you to ride thermals all the way to the top.