Hello,
When loading in a large level into an already potentially large persistent level, it is definitely possible that there will be some negative impacts on performance. It may be a good idea to rethink whether or not level streaming is best for your current design, as from what you say it seems that you are attempting to stream in quite a large and detailed level.
If you are intent on using streaming in your game, ensure that your levels are as optimized as possible. Below are links to our documentation on Level Streaming, as well as a short tutorial that shows a basic example of Level Streaming at work:
Finally, have a look at the Content Examples project that is located in the Learn tab of the Launcher. In it, you will find an entire map dedicated to level streaming that showcases the process as well as provides examples of Level Streaming at work.
Have a great day