Judging by how Intel processors work, I'm assuming Athlons do the same thing. When the CPU is overheating, it will reduce its CPU speed to compensate, which makes perfect sense from the screenshots you provided. So even at a lower clock speed, it was getting high temperatures. Once you cleared out the vents, airflow increased, lowering the temperature, and the CPU was able to work at a higher speed.
Rule of thumb, especially for laptops: always make sure those vents are free of dust and debris.