I too had a Thinkpad T400, in about 2013. Compiling my undergrad university course code made the music I was listening to skip.
Modern software sucks, I can not imagine using a core2duo to do any task other than ssh/word processing
I was using a W500 (same generation) until 2018 or so. Upgraded RAM, installed an SSD, and it was my daily driver until the end. But already back then it started to become unbearbly slow especially with background procesesses like file syncing to different machines.