The internal flash is usually way faster than an SD-card. In your example, the Samsung Galaxy S10 uses UFS 2.1 storage for interfacing with the internal flash chip.
UFS 2.1 for the 1TB storage configuration you can expect in reality about (maximum values):
- 1000 MB/sec sequential write speed
- 260 MB/sec sequential write speed
- 50000-58000 IOPS
If you try to get a micro SD-card, there are a few that are as fast as the internal storage, but they use the UHS-II standard or newer (max ~620 MB/sec). For UHS-II, the micro SD-card reader in the phone needs additional contact points, but only recent high-end phones have such a reader integrated (I have read that the Samsung Note 10 released 2019 was the first Samsung phone to have such a reader).
Another problem of micro SD-cards is their size. The faster cards require a lot of CPU speed of the CPU integrated in the card. Because of the small size the card is very limited regarding dispensing the created heat. If you for example use such a micro SD-card in a PC card reader the micro SD card is getting hot very fast and then starts throttling the read/write speed.
But development of UFS also does not stop. Just recently, UFS 4.0 has been announced which can reach transfer speeds that are common for NMVe SSDs in a PC (up to 5GB/sec).
Therefore, for most micro SD cards, I would expect that speed of a combined set-up of integrated storage + a quality microSD will not be faster that the internal flash chip alone. Because of the latency of the micro SD-card, it might be even slower (but that may depend on the used phone and the micro SD-card).