Hello. Manual modification within the ALM database tables is not recommended or supported by MF, as this may impact the projects functionality. Since you have so many test runs to delete, I would like to suggest the following utility to delete the old test runs from ALM. The Stand Alone ALM Purge Utility should be able to successfully delete the old test runs without sacrificing machine performance. The following link will take you to a knowledge document where the utility can be downloaded. The knowledge document also contains information on how to use the utility. Before using the tool, be sure to make a backup copy of the project you wish to use the tool on, in the case of any unforeseen issue occurs. I hope this helps!
[Link to the Stand-Alone ALM Purge Utility]