So lets say that the level of the non-magic wielding species i.e. humans technology is at around the level it was in the early medieval times.
I would then introduce magic to the rest of the world in a form of a new species. The species i.e. elves would have lived within the world but hidden from the humankind. They would have long traditions with magic etc. The whole race would be peaceful (with a few exceptions).
The magic would lurk in to the world as humanity knows it, through some elves sent to live among them. And the country/ area of elves would become open for trade etc. (All of this due to the elves needing more resources etc. to live in the small area with a growing population)
There would be a few very strong elves who wouldn't have many limits on the magic, but those few would have more like a god status and have little to no interest in the happenings of this world.
Then there would be also a few powerful elves whose magic is strong enough to be the leaders of the elf country. They'd be peaceful and only use their powers when needed to defend their kind. If there was a war they would be easily the most powerful country in the world.
The average towns folk elf would be powerful compared to humans, but they wouldn't have so many spells or types of magic they can control. I.e. one could hold about 5 different spells which would be directed toward their professions and learned with long training. So lets say there was a farmer elf. That elf would probably have spells related to water, growing plants, shaping of land and around two others. On top of the main spells they would have learned a lot of weak spells etc.
Wielding magic without any magical items would still be very rare in the human countries and the average human could only learn to wield it through certain life threatening rituals and years of studying and training.
There would however, be many magic items introduced to humans trough the trade some countries open with the elves i.e. potions, weapons and lets say crystals that hold magical energy (would be needed to use magical weapons) and many other types of things. These items would be "low in magic" making the elves and few people have the real power of magic.
Magic would (very) slowly become inherited(?) to the mankind through slow mixing of the species (which would be banned at places -> very slow). It would however make the powerful magic wielders increase with time.
Problem comes with the few master crafts elves and weapon makers etc. who'd be able to make guns or something like that. And these could "modernize the world overnight" so to say.
The problem what this makes, is that (what I call) the technology level of the world would eventually increase its developing speed when I want to keep the human side of worlds era in the middle-ages (and generally not modernized) as long or possibly longer than it was in the Earth.
I really don't want the world to get to modernity any time soon, so my question is:
How do I keep the (mainly human) world from starting to develop their technology level too fast or "modernizing overnight" with the introduction of magic?