There are multiple things involved with your question. I know this topic is hard to put in exact words, especially for someone new to Android -- so it gets a little unclear what the real issue is. I'll try to cover all I think is related, but will redirect to other questions here for details.
First: How RAM is used.
There is no such thing like "useful free RAM". As soon as there's "free RAM" available, it will be put to use: buffering and caching are the most interesting features in this context, as they speed up the system. As soon as an app requires additional RAM, and there's no free RAM available, the required amount is dropped from caches and buffers, so this is entirely dynamic and should have no negative effects.
More details:
There are a lot more questions here dealing with this topic. To find them all, just follow the ram tag you already used for your question.
Second: Your device slowing down.
This might have many different reasons: Some app might be hogging ressources is the most likely reason if the problem is not there right after a reboot (but starts only a few hours/days later). If the problem persists after a reboot, reasons might be different (or the "hogging app" got auto-started). Your caches (on internal storage) could be messed up, for example. If you installed/uninstalled a lot of apps for testing, it could as well be the Dalvik-Cache (pre-compiled code of your apps).
Good starting point to investigate this issue:
My device is getting slow, apps start misbehaving/crashing. What can I do?
See also the "Linked" and "Related" section of that question, which will provide you additional ressources.
