At 12 years old it's still worth saving in most cases. The problem of heating slowly is often a clogged burner or weak heating element, and that's way cheaper than a whole new oven. Before calling a technician again, try cleaning the burner really well with a fine needle in the little holes - residue builds up there and it really affects performance. If it still doesn't improve after that, then it's worth getting a repair quote.
The thing is knowing what the previous technician tried to do. If he just cleaned it and charged you a lot, maybe it wasn't anything serious. But if the problem is actually in the heating element or thermostat, you have to decide: repair costs about half the price of a new oven, so it's worth it. Only if it was something structural with the oven itself would it not be worth it.
My practical tip: when you turn on the oven, put a piece of paper or a cup of water inside for a minute and see if it heats up fast. If the paper gets yellowed or the water steams up quickly, the oven is working - it might just be a matter of cleaning or calibration. If nothing happens, that's a sign the heating element is actually weak and you need to get a quote.