Complex sentence: By 1913 his "big boot dance" was becoming mentally and physically challenging for him and so he abandoned it in 1915, concentrating instead on his comic sketches involving the characters the Tram Conductor, the Gas Inspector, and the Spanish Senora.
Split into simpler sentences: By 1913 his "big boot dance" was becoming too mentally and physically challenging for him, causing him to abandon it in 1915. From then on, he concentrated instead on his comic sketches involving his characterisations of the Tram Conductor, the Gas Inspector, and the Spanish Senora.