Hi Stefanie,
As mentioned by you, there are lot of jobs running.
So I can say you may also have to check on many other possibility,
1. If there is many jobs using same resource in parallel, In that case one job waiting for other job to complete.
2. Also what is the time duration of each job. Whether your transaction is able to finish that job within that time frame. If not this may put the other jobs on wait and may go to next schedule and you can see lot of delay.
3. Database size has become very high so any insert or update or select operation may be taking more time.
4. In any RFC interface jobs check whether you are taking status back to MII after the successful execution of RFC.
You can make use of sm50 Tcode to see the running jobs in ERP, so that you can analyse what time it is taking.
I suggest to raise an OSS ticket so that you can get quick fix.
Thanks,
Padma Rao