Classic model belt problem

I just bought a used classic model and i had to replace the engine to transmission belt and the engine to deck belt. Before I replaced the belts the operating rpm was 3750 sitting, 3720 to 3740 with the deck engaged, and stayed above 3700 when cutting at full speed with moderate grass height. With the new belts on, sitting rpm is 3710 to 3700, deck engaged 3640 to 3690, and when mowing it drops below 3640 to 3570. It is not the world's fastest mower anymore. The engine bogs down through moderate grass height.

I am going to loosen the engine mounts tomorrow to relieve some belt tension. Does this sound like it could be the problem.

Thanks, Bill

Info for anyone that may need it.

The problem with the engine RPMs was in fact the belts were too tight. This is what I did, I loosened the engine mounting bolts and slid the engine all the way forward to remove both transmission and engine to deck belt. I then started the engine and let it run for a few mins to see what the rpms were with absolutely no tension on the engine (3880 rpms). I then added the transmission belt and adjusted the engine and the tensioner pulley to obtain 3750- 3770 rpms. Then I added the engine to deck belt and adjusted the spring tension to 1.75" to 2" for my model year 2007. I went and cut some grass and although she did bog down in the tall thick grass, which I expected, but ran great everywhere else. I think I will tinker with the tension a little more (minor) and set and forget about it.