Can indexes really decrease performances in a low volume writes DB?
You have a lot of questions and unknowns (no info on the size of db, hardware - RAM, CPU, Disk (san or local ssd), is the server VM or physical, do you have baseline as to what looks good or what is acceptable, sp_configure settings, do you run maintenance - update stats, index reorg/rebuild etc) in your question. 


  should I go on and blindly add indexes


No. Read this first - Don't just blindly create those "missing" indexes!


Once you read that, do a health check sp_Blitz: Overall Health Check
Review and Address the high priority ones.
Tune your indexes - sp_BlitzIndex: Tune Your Indexes


Above will get you in good shape. If you see any issues, post the question back and we will help you :-)