A microSD-to-SD adapter is normally just a passive wiring adapter, so it should not meaningfully reduce write speed. In practice, the card’s own write performance is the important factor.

So yes: if your current card is too slow for video, a faster microSD card used in the same adapter should help, assuming the card actually meets the camera’s required sustained write speed.

That said, some photographers avoid adapters in cameras because they add one more potential point of failure. Possible issues include poor contact, adapter wear, or in rare cases the adapter getting damaged or stuck. For reliability, a full-size SD card is often the safer choice if your camera accepts SD directly.

In short: the adapter itself usually isn’t the speed bottleneck; the microSD card speed is.