What's the meaning of ORM?

You have mostly answered your own question when you listed the benefits of an ORM. There are definitely some optimisation issues that you will encounter but the abstraction of the database interface probably over-rides these downsides.

You mention that the ORM sometimes uses many sql statements where it could use only one. You may want to look at "eager loading", if this is supported by your ORM. This tells the ORM to fetch the data from related models at the same time as it fetches data from another model. This should result in more performant sql.

I would suggest that you stick with your ORM and optimise the parts that need it, but, explore any methods within the ORM that allow you to increase performance before reverting to writing SQL to do the access.

A good ORM allows you to tune the data access if you discover that certain queries are a bottleneck.

But the fact that you might need to do this does not in any way remove the value of the ORM approach, because it rapidly gets you to the point where you can discover where the bottlenecks are. It is rarely the case that every line of code needs the same amount of careful hand-optimisation. Most of it won't. Only a few hotspots require attention.

If you write all the SQL by hand, you are "micro optimising" across the whole product, including the parts that don't need it. So you're mostly wasting effort.

here is the definition from Wikipedia

Object-relational mapping is a programming technique for converting data between incompatible type systems in relational databases and object-oriented programming languages. This creates, in effect, a "virtual object database" that can be used from within the programming language.