i'm quite new to bitcoin, so a lot of questions came to my mind. As i do not own BTC yet, i'm quite unsure if it is the right moment to invest, as - from my own perspective - the BTC has been hyped heavily in 2017 as "regular people" media has picked up the topic and a lot of people invested.
on the other hand there are technical limitations which are the transaction time and the high fees. could this lead to a heavy decrease in value or are you thinking BTC will continue to increase in value?
You've already answered your question: even the normal people already invested in bitcoin thus making the value of it increase over time. Long transaction times and high fees are not technical limitations but rather a hurdle that is understandable to encounter along the way as the protocol handles more and more volume every day (though it is annoying honestly). There will come a time that all these issues would be addressed hopefully by the lightning network.
as far as i know there are no smart contracts possible on the btc blockchain.
also the scalability is limited.
what is the lightning network?
Bitcoin is meant to be the superhighway of cheap and fast transactions and not smart contracts. You have other coins for that matter like ETH which is its signature edge. Also, the lightning network is the proposed scaling solution for bitcoin in which transactions will happen off-chain resulting into a fast and affordable transaction. It could turn out good/bad but again, that is until we tested the whole thing real time where everyone is using bitcoin for transfers.