Our experience is the opposite, we have a pretty large flow typed code base, and can do a full check in <100ms. When we converted to TS (decided not to merged) we saw typescript was in the multiple minute mark. It’s worth checking out LTI and how the typing on boundaries, enables flow to parallelize and give very precise error messages compared to TS. The third party lib support is however basically dead, except the latest versions of flow are starting to enable ingestion of TS types, so that’s interesting.