any ideas what it is and how to stop it?

Caterpillars? They can move pretty quickly but have you looked at the rear side of the leaves? Some appear only at night. Most insecticides will deal with caterpillars. Sytsemic insecticide would be advisable

 

Looks like a fungal leaf spot. At first I thought it was caterpillar feeding (moth larvae) but the spot in the lower right of the photo still has the brown interior necrotic tissue and the other holes have necrotic margins, not clean chewing edges. Weekly spraying with neem oil would cover both possibilities, pest and disease, without toxicity. When problem clears up on new growth, you can reduce frequency of sprays to two weeks. If it's a fungal pathogen it can be related to wet season or overhead watering. During dry season you may not have to spray at all. 

Just make sure you don't have any of these little bas*&^ds:

 

 

Take a sample of the affected leaves to one of the small shops that sell garden fertilizers etc., there's loads of them on the outskirts of CM, particularly in Mae Rim, alternatively take the sample to the garden centre next to  JJ's market. We have a small shop nearby that we use for this purpose, the girl who runs it studied agriculture at Mae Jo Uni. and is very good at diagnosis of soil/bugs et al.  We have a large garden with over 35 trees and we've been hit this year with similar (but different) types of problems to the one you describe. For one of the problems there was no choice other than to use a broad spectrum bug killer (bad because it kills everything) that must be sprayed on, others require less radical solutions such as spraying on a combination of things that deter the various bugs from eating the leaves. Friends and nearby neighbours swear blind that spraying the leaves once a month with a combination of chillies and garlic (soaked for a few days before hand) stops any bugs from eating the leaves - I've yet to try it.

There is always a choice to using broad spectrum bug killers. You have presented one of the many botanical pest suppression options. But if you don't try it you will never know.

Tip: Use botanicals preventively, or at early onset of infestations. If you wait until your plants have advanced infestations you will have more trouble controlling pests without the harsher chemicals. Get in touch with the cycles in your garden and the repeat offenders. The specific pests you have at this time of year will probably return at pretty much the same time next year.  Keep a calendar.  Go with a pre-emptive strike; start with repellant botanicals early, before they get started. Watch for moths flying around and consider that they may be laying eggs that will hatch into hungry caterpillars. Watch for beetles and weevils starting to feed. Look on the underside of leaves for aphids, and on leaves, petioles and stems for scale and mealybugs. Be ready with some basic materials and a good sprayer so you don't have to take time to go shopping while the pest population is building up. Hort oil spray in winter can help with preventive pest management. Insecticidal soap and pyrethrins are good for knock down. Neem oil and azadirachtin products are good repellents, anti-feedants and reproductive disruptors. Neem oil can suppress certain plant pathogens too.