Hi Uwe, a couple you might want to consider depending on the situation (e.g., are consultants performing in staff aug role or doing true short-term consulting projects) might be: 1.) retention of key personnel; 2.) attrition / turnover; 3.) onboarding time (much of this is client-dependent); 4.) required training required before billing starts.
• SimCorp A/S
You can also add customer escalations for individual performance issues. If the consulting project has delivery responsibility at end of project on T&M, then SLA can be that vendor's actual efforts deviation should not be more than 10% of the initial estimations. Or similar others on performance or outcome based.