When you modify intersections with Lane connector tool all vehicles incoming to it will need to request new path. If there is a long queue of path requests (enable pathfinding status at Maintenance tab in the options so see how long it is). If the number under TMPE main menu window is high (>100-200) or worse, constantly rising it means that either your CPU has too few cores to be used as additional Path finding threads or somewhere on the map citizens stuck at intersection - they literally flood pathfinding process with "get new path" requests and Pathfinding is struggling with handling such amount of requests (walking routes might be really long so calculating each takes more time).