The Turbo-XT bios I use has options to leave the turbo on or off during disk-access. Might be a good idea to test.
https://www.phatcode.net/downloads.php?id=101
I just got a Toshiba T-1500 desktop PC/XT. Unfortunately the only info I found on the web is this:
http://www.todocoleccion.net/catalogo-informatica-ordenador-toshiba-t1500-anos-80-30-x-21-desplegado-60-x-21-muy-raro~x18475339
Specs:
4.77MHz 8088 (white ceramic Fairchild, soldered)
640k RAM...