Complex sentence: High speed controls were light thus high speed turns and pull-outs were possible although the P-39 had to be held in a dive since it tended to level out, reminiscent of the Spitfire.
Split into simpler sentences: High speed controls were light; thus, high speed turns and pull-outs were possible. However, the P-39 had to be held in a dive since it tended to level out, reminiscent of the Spitfire.