Is it "necessary" to switch from RPI3B to RPI4B?

?? You don’t run VS on your OH box so I don’t get that point. Do you mean to make excessive use of the LSP service ?

You probably didn’t optimize your install for memory use, that’s where additional RAM can make for a big difference. But that’s untrue as a general statement.

It’s dangerously wrong to assume eMMC is better than SD so please don’t keep recommending this.
Better go ZRAM.
And under normal usage conditions, no openHAB user will ever need nor take advantage of the bandwidth the interface provides so ultimately there’s no serious real advantage left why to go with an Odroid while you for sure do get all the disadvantages because it just isn’t the mainstream that RPis represent.
But keep general discussion in the appropriate thread(s), please.
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