The numbers you posted sound normal to me for SQL Server. It sounds to me like you don't understand how SQL Server uses memory. Take a look through my blog post:

Troubleshooting the SQL Server Memory Leak (or Understanding SQL Server Memory Usage)

If you don't want SQL Server to use all of the available memory, then you need to configure the max server memory setting to control how much it can use. Slow inserts won't be associated with SQL Server using memory, in fact reducing memory to SQL can actually cause further problems with performance of inserts since you are going to force SQL Server to have to do more hard IO to disk which impacts write performance as a side effect.