I tend to be in trades with max 2 assets at a time, I find it hard to keep track of more. I trade when there is a setup I like rather than because it is a particular time of day.
if you consider a faster and an equivalent slower market, say DAX and Euro Stoxx 50, which one would you trade and which you just observe and use as a hint for the market you trade?
I don't trade those assets. But in markets that move closely together, e.g. if I were trading BTC and LTC then if the leader breaks I may jump on the other expecting it to follow. Similarly if the smaller asset breaks I may not jump in unless the leader breaks, to avoid a false breakout. For US Equities I may watch QQQ and XLF if I'm trading SPY, or I may watch SPY if I'm trading BA, or I may watch AAPL if I'm trading NVDA. It all depends on the situation and how the market has been reacting recently.