Lwt_preemptive.detach forces a full major GC run every 2500 calls at most

[mfp]

Lwt_preemptive uses Event channels which under the hood allocate one condition variable per Event.sync. The parameters given to caml_alloc_custom when creating the condition variable indicate that at most 5000 values can be left unreclaimed. Heavy GC load ensues.

See mfp/ocaml-sqlexpr#13

Also, should a (performance) bug be filed against OCaml's Event module?